diff --git a/src/rosbags/serde/cdr.py b/src/rosbags/serde/cdr.py
index 8dc87c96..894cc5fc 100644
--- a/src/rosbags/serde/cdr.py
+++ b/src/rosbags/serde/cdr.py
@@ -44,7 +44,7 @@ def generate_getsize_cdr(fields: list[Field]) -> tuple[CDRSerSize, int]:
     lines = [
         'import sys',
         'from rosbags.serde.messages import get_msgdef',
-        'def getsize_cdr(pos, message):',
+        'def getsize_cdr(pos, message, typestore):',
     ]
     for fcurr, fnext in zip(icurr, inext):
         fieldname, desc = fcurr
@@ -54,8 +54,8 @@ def generate_getsize_cdr(fields: list[Field]) -> tuple[CDRSerSize, int]:
                 lines.append(f'  pos += {desc.args.size_cdr}')
                 size += desc.args.size_cdr
             else:
-                lines.append(f'  func = get_msgdef("{desc.args.name}").getsize_cdr')
-                lines.append(f'  pos = func(pos, message.{fieldname})')
+                lines.append(f'  func = get_msgdef("{desc.args.name}", typestore).getsize_cdr')
+                lines.append(f'  pos = func(pos, message.{fieldname}, typestore)')
                 is_stat = False
             aligned = align_after(desc)
 
@@ -97,12 +97,14 @@ def generate_getsize_cdr(fields: list[Field]) -> tuple[CDRSerSize, int]:
                         lines.append(f'  pos += {subdesc.args.size_cdr}')
                         size += subdesc.args.size_cdr
                 else:
-                    lines.append(f'  func = get_msgdef("{subdesc.args.name}").getsize_cdr')
+                    lines.append(
+                        f'  func = get_msgdef("{subdesc.args.name}", typestore).getsize_cdr',
+                    )
                     lines.append(f'  val = message.{fieldname}')
                     for idx in range(length):
                         if anext_before > anext_after:
                             lines.append(f'  pos = (pos + {anext_before} - 1) & -{anext_before}')
-                        lines.append(f'  pos = func(pos, val[{idx}])')
+                        lines.append(f'  pos = func(pos, val[{idx}], typestore)')
                     is_stat = False
                 aligned = align_after(subdesc)
         else:
@@ -138,13 +140,15 @@ def generate_getsize_cdr(fields: list[Field]) -> tuple[CDRSerSize, int]:
                     lines.append(f'    pos += {subdesc.args.size_cdr}')
 
                 else:
-                    lines.append(f'  func = get_msgdef("{subdesc.args.name}").getsize_cdr')
+                    lines.append(
+                        f'  func = get_msgdef("{subdesc.args.name}", typestore).getsize_cdr',
+                    )
                     if aligned < anext_before <= anext_after:
                         lines.append(f'  pos = (pos + {anext_before} - 1) & -{anext_before}')
                     lines.append('  for item in val:')
                     if anext_before > anext_after:
                         lines.append(f'    pos = (pos + {anext_before} - 1) & -{anext_before}')
-                    lines.append('    pos = func(pos, item)')
+                    lines.append('    pos = func(pos, item, typestore)')
                 aligned = align_after(subdesc)
 
             aligned = min([aligned, 4])
@@ -190,15 +194,16 @@ def generate_serialize_cdr(fields: list[Field], endianess: str) -> CDRSer:
         f'from rosbags.serde.primitives import pack_uint64_{endianess}',
         f'from rosbags.serde.primitives import pack_float32_{endianess}',
         f'from rosbags.serde.primitives import pack_float64_{endianess}',
-        'def serialize_cdr(rawdata, pos, message):',
+        'def serialize_cdr(rawdata, pos, message, typestore):',
     ]
     for fcurr, fnext in zip(icurr, inext):
         fieldname, desc = fcurr
 
         lines.append(f'  val = message.{fieldname}')
         if desc.valtype == Valtype.MESSAGE:
-            lines.append(f'  func = get_msgdef("{desc.args.name}").serialize_cdr_{endianess}')
-            lines.append('  pos = func(rawdata, pos, val)')
+            name = desc.args.name
+            lines.append(f'  func = get_msgdef("{name}", typestore).serialize_cdr_{endianess}')
+            lines.append('  pos = func(rawdata, pos, val, typestore)')
             aligned = align_after(desc)
 
         elif desc.valtype == Valtype.BASE:
@@ -242,13 +247,12 @@ def generate_serialize_cdr(fields: list[Field], endianess: str) -> CDRSer:
                 assert subdesc.valtype == Valtype.MESSAGE
                 anext_before = align(subdesc)
                 anext_after = align_after(subdesc)
-                lines.append(
-                    f'  func = get_msgdef("{subdesc.args.name}").serialize_cdr_{endianess}',
-                )
+                name = subdesc.args.name
+                lines.append(f'  func = get_msgdef("{name}", typestore).serialize_cdr_{endianess}')
                 for idx in range(length):
                     if anext_before > anext_after:
                         lines.append(f'  pos = (pos + {anext_before} - 1) & -{anext_before}')
-                    lines.append(f'  pos = func(rawdata, pos, val[{idx}])')
+                    lines.append(f'  pos = func(rawdata, pos, val[{idx}], typestore)')
                 aligned = align_after(subdesc)
         else:
             assert desc.valtype == Valtype.SEQUENCE
@@ -281,12 +285,11 @@ def generate_serialize_cdr(fields: list[Field], endianess: str) -> CDRSer:
 
             if subdesc.valtype == Valtype.MESSAGE:
                 anext_before = align(subdesc)
-                lines.append(
-                    f'  func = get_msgdef("{subdesc.args.name}").serialize_cdr_{endianess}',
-                )
+                name = subdesc.args.name
+                lines.append(f'  func = get_msgdef("{name}", typestore).serialize_cdr_{endianess}')
                 lines.append('  for item in val:')
                 lines.append(f'    pos = (pos + {anext_before} - 1) & -{anext_before}')
-                lines.append('    pos = func(rawdata, pos, item)')
+                lines.append('    pos = func(rawdata, pos, item, typestore)')
                 aligned = align_after(subdesc)
 
             aligned = min([4, aligned])
@@ -330,7 +333,7 @@ def generate_deserialize_cdr(fields: list[Field], endianess: str) -> CDRDeser:
         f'from rosbags.serde.primitives import unpack_uint64_{endianess}',
         f'from rosbags.serde.primitives import unpack_float32_{endianess}',
         f'from rosbags.serde.primitives import unpack_float64_{endianess}',
-        'def deserialize_cdr(rawdata, pos, cls):',
+        'def deserialize_cdr(rawdata, pos, cls, typestore):',
     ]
 
     funcname = f'deserialize_cdr_{endianess}'
@@ -339,8 +342,8 @@ def generate_deserialize_cdr(fields: list[Field], endianess: str) -> CDRDeser:
         desc = fcurr[1]
 
         if desc.valtype == Valtype.MESSAGE:
-            lines.append(f'  msgdef = get_msgdef("{desc.args.name}")')
-            lines.append(f'  obj, pos = msgdef.{funcname}(rawdata, pos, msgdef.cls)')
+            lines.append(f'  msgdef = get_msgdef("{desc.args.name}", typestore)')
+            lines.append(f'  obj, pos = msgdef.{funcname}(rawdata, pos, msgdef.cls, typestore)')
             lines.append('  values.append(obj)')
             aligned = align_after(desc)
 
@@ -386,12 +389,14 @@ def generate_deserialize_cdr(fields: list[Field], endianess: str) -> CDRDeser:
                 assert subdesc.valtype == Valtype.MESSAGE
                 anext_before = align(subdesc)
                 anext_after = align_after(subdesc)
-                lines.append(f'  msgdef = get_msgdef("{subdesc.args.name}")')
+                lines.append(f'  msgdef = get_msgdef("{subdesc.args.name}", typestore)')
                 lines.append('  value = []')
                 for _ in range(length):
                     if anext_before > anext_after:
                         lines.append(f'  pos = (pos + {anext_before} - 1) & -{anext_before}')
-                    lines.append(f'  obj, pos = msgdef.{funcname}(rawdata, pos, msgdef.cls)')
+                    lines.append(
+                        f'  obj, pos = msgdef.{funcname}(rawdata, pos, msgdef.cls, typestore)',
+                    )
                     lines.append('  value.append(obj)')
                 lines.append('  values.append(value)')
                 aligned = align_after(subdesc)
@@ -433,11 +438,13 @@ def generate_deserialize_cdr(fields: list[Field], endianess: str) -> CDRDeser:
 
             if subdesc.valtype == Valtype.MESSAGE:
                 anext_before = align(subdesc)
-                lines.append(f'  msgdef = get_msgdef("{subdesc.args.name}")')
+                lines.append(f'  msgdef = get_msgdef("{subdesc.args.name}", typestore)')
                 lines.append('  value = []')
                 lines.append('  for _ in range(size):')
                 lines.append(f'    pos = (pos + {anext_before} - 1) & -{anext_before}')
-                lines.append(f'    obj, pos = msgdef.{funcname}(rawdata, pos, msgdef.cls)')
+                lines.append(
+                    f'    obj, pos = msgdef.{funcname}(rawdata, pos, msgdef.cls, typestore)',
+                )
                 lines.append('    value.append(obj)')
                 lines.append('  values.append(value)')
                 aligned = align_after(subdesc)
diff --git a/src/rosbags/serde/messages.py b/src/rosbags/serde/messages.py
index bad20c4e..c4f52097 100644
--- a/src/rosbags/serde/messages.py
+++ b/src/rosbags/serde/messages.py
@@ -6,8 +6,6 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-from rosbags.typesys import types
-
 from .cdr import generate_deserialize_cdr, generate_getsize_cdr, generate_serialize_cdr
 from .ros1 import generate_cdr_to_ros1, generate_ros1_to_cdr
 from .typing import Descriptor, Field, Msgdef
@@ -15,28 +13,34 @@ from .utils import Valtype
 
 if TYPE_CHECKING:
     from rosbags.typesys.base import Fielddesc
+    from rosbags.typesys.register import Typestore
 
-MSGDEFCACHE: dict[str, Msgdef] = {}
+MSGDEFCACHE: dict[Typestore, dict[str, Msgdef]] = {}
 
 
 class SerdeError(Exception):
     """Serialization and Deserialization Error."""
 
 
-def get_msgdef(typename: str) -> Msgdef:
+def get_msgdef(typename: str, typestore: Typestore) -> Msgdef:
     """Retrieve message definition for typename.
 
     Message definitions are cached globally and generated as needed.
 
     Args:
         typename: Msgdef type name to load.
+        typestore: Type store.
 
     Returns:
         Message definition.
 
     """
-    if typename not in MSGDEFCACHE:
-        entries = types.FIELDDEFS[typename][1]
+    if typestore not in MSGDEFCACHE:
+        MSGDEFCACHE[typestore] = {}
+    cache = MSGDEFCACHE[typestore]
+
+    if typename not in cache:
+        entries = typestore.FIELDDEFS[typename][1]
 
         def fixup(entry: Fielddesc) -> Descriptor:
             if entry[0] == Valtype.BASE:
@@ -44,7 +48,7 @@ def get_msgdef(typename: str) -> Msgdef:
                 return Descriptor(Valtype.BASE, entry[1])
             if entry[0] == Valtype.MESSAGE:
                 assert isinstance(entry[1], str)
-                return Descriptor(Valtype.MESSAGE, get_msgdef(entry[1]))
+                return Descriptor(Valtype.MESSAGE, get_msgdef(entry[1], typestore))
             if entry[0] == Valtype.ARRAY:
                 assert not isinstance(entry[1][0], str)
                 return Descriptor(Valtype.ARRAY, (fixup(entry[1][0]), entry[1][1]))
@@ -59,10 +63,10 @@ def get_msgdef(typename: str) -> Msgdef:
 
         getsize_cdr, size_cdr = generate_getsize_cdr(fields)
 
-        MSGDEFCACHE[typename] = Msgdef(
+        cache[typename] = Msgdef(
             typename,
             fields,
-            getattr(types, typename.replace('/', '__')),
+            getattr(typestore, typename.replace('/', '__')),
             size_cdr,
             getsize_cdr,
             generate_serialize_cdr(fields, 'le'),
@@ -74,4 +78,4 @@ def get_msgdef(typename: str) -> Msgdef:
             generate_cdr_to_ros1(fields, typename, False),  # type: ignore
             generate_cdr_to_ros1(fields, typename, True),  # type: ignore
         )
-    return MSGDEFCACHE[typename]
+    return cache[typename]
diff --git a/src/rosbags/serde/ros1.py b/src/rosbags/serde/ros1.py
index 402afb8c..38322e4c 100644
--- a/src/rosbags/serde/ros1.py
+++ b/src/rosbags/serde/ros1.py
@@ -52,7 +52,7 @@ def generate_ros1_to_cdr(
         'from rosbags.serde.messages import SerdeError, get_msgdef',
         'from rosbags.serde.primitives import pack_int32_le',
         'from rosbags.serde.primitives import unpack_int32_le',
-        f'def {funcname}(input, ipos, output, opos):',
+        f'def {funcname}(input, ipos, output, opos, typestore):',
     ]
 
     if typename == 'std_msgs/msg/Header':
@@ -62,8 +62,8 @@ def generate_ros1_to_cdr(
         _, desc = fcurr
 
         if desc.valtype == Valtype.MESSAGE:
-            lines.append(f'  func = get_msgdef("{desc.args.name}").{funcname}')
-            lines.append('  ipos, opos = func(input, ipos, output, opos)')
+            lines.append(f'  func = get_msgdef("{desc.args.name}", typestore).{funcname}')
+            lines.append('  ipos, opos = func(input, ipos, output, opos, typestore)')
             aligned = align_after(desc)
 
         elif desc.valtype == Valtype.BASE:
@@ -117,11 +117,11 @@ def generate_ros1_to_cdr(
                 anext_before = align(subdesc)
                 anext_after = align_after(subdesc)
 
-                lines.append(f'  func = get_msgdef("{subdesc.args.name}").{funcname}')
+                lines.append(f'  func = get_msgdef("{subdesc.args.name}", typestore).{funcname}')
                 for _ in range(length):
                     if anext_before > anext_after:
                         lines.append(f'  opos = (opos + {anext_before} - 1) & -{anext_before}')
-                    lines.append('  ipos, opos = func(input, ipos, output, opos)')
+                    lines.append('  ipos, opos = func(input, ipos, output, opos, typestore)')
                 aligned = anext_after
         else:
             assert desc.valtype == Valtype.SEQUENCE
@@ -163,10 +163,10 @@ def generate_ros1_to_cdr(
             else:
                 assert subdesc.valtype == Valtype.MESSAGE
                 anext_before = align(subdesc)
-                lines.append(f'  func = get_msgdef("{subdesc.args.name}").{funcname}')
+                lines.append(f'  func = get_msgdef("{subdesc.args.name}", typestore).{funcname}')
                 lines.append('  for _ in range(size):')
                 lines.append(f'    opos = (opos + {anext_before} - 1) & -{anext_before}')
-                lines.append('    ipos, opos = func(input, ipos, output, opos)')
+                lines.append('    ipos, opos = func(input, ipos, output, opos, typestore)')
                 aligned = align_after(subdesc)
 
             aligned = min([aligned, 4])
@@ -208,7 +208,7 @@ def generate_cdr_to_ros1(
         'from rosbags.serde.messages import SerdeError, get_msgdef',
         'from rosbags.serde.primitives import pack_int32_le',
         'from rosbags.serde.primitives import unpack_int32_le',
-        f'def {funcname}(input, ipos, output, opos):',
+        f'def {funcname}(input, ipos, output, opos, typestore):',
     ]
 
     if typename == 'std_msgs/msg/Header':
@@ -218,8 +218,8 @@ def generate_cdr_to_ros1(
         _, desc = fcurr
 
         if desc.valtype == Valtype.MESSAGE:
-            lines.append(f'  func = get_msgdef("{desc.args.name}").{funcname}')
-            lines.append('  ipos, opos = func(input, ipos, output, opos)')
+            lines.append(f'  func = get_msgdef("{desc.args.name}", typestore).{funcname}')
+            lines.append('  ipos, opos = func(input, ipos, output, opos, typestore)')
             aligned = align_after(desc)
 
         elif desc.valtype == Valtype.BASE:
@@ -273,11 +273,11 @@ def generate_cdr_to_ros1(
                 anext_before = align(subdesc)
                 anext_after = align_after(subdesc)
 
-                lines.append(f'  func = get_msgdef("{subdesc.args.name}").{funcname}')
+                lines.append(f'  func = get_msgdef("{subdesc.args.name}", typestore).{funcname}')
                 for _ in range(length):
                     if anext_before > anext_after:
                         lines.append(f'  ipos = (ipos + {anext_before} - 1) & -{anext_before}')
-                    lines.append('  ipos, opos = func(input, ipos, output, opos)')
+                    lines.append('  ipos, opos = func(input, ipos, output, opos, typestore)')
                 aligned = anext_after
         else:
             assert desc.valtype == Valtype.SEQUENCE
@@ -317,10 +317,10 @@ def generate_cdr_to_ros1(
             else:
                 assert subdesc.valtype == Valtype.MESSAGE
                 anext_before = align(subdesc)
-                lines.append(f'  func = get_msgdef("{subdesc.args.name}").{funcname}')
+                lines.append(f'  func = get_msgdef("{subdesc.args.name}", typestore).{funcname}')
                 lines.append('  for _ in range(size):')
                 lines.append(f'    ipos = (ipos + {anext_before} - 1) & -{anext_before}')
-                lines.append('    ipos, opos = func(input, ipos, output, opos)')
+                lines.append('    ipos, opos = func(input, ipos, output, opos, typestore)')
                 aligned = align_after(subdesc)
 
             aligned = min([aligned, 4])
diff --git a/src/rosbags/serde/serdes.py b/src/rosbags/serde/serdes.py
index 96d4aabe..6d56a7b0 100644
--- a/src/rosbags/serde/serdes.py
+++ b/src/rosbags/serde/serdes.py
@@ -8,18 +8,27 @@ import sys
 from struct import pack_into
 from typing import TYPE_CHECKING
 
+from rosbags.typesys import types
+
 from .messages import get_msgdef
 
 if TYPE_CHECKING:
     from typing import Any
 
+    from rosbags.typesys.register import Typestore
 
-def deserialize_cdr(rawdata: bytes, typename: str) -> Any:  # noqa: ANN401
+
+def deserialize_cdr(
+    rawdata: bytes,
+    typename: str,
+    typestore: Typestore = types,
+) -> Any:  # noqa: ANN401
     """Deserialize raw data into a message object.
 
     Args:
         rawdata: Serialized data.
         typename: Message type name.
+        typestore: Type store.
 
     Returns:
         Deserialized message object.
@@ -27,9 +36,9 @@ def deserialize_cdr(rawdata: bytes, typename: str) -> Any:  # noqa: ANN401
     """
     little_endian = bool(rawdata[1])
 
-    msgdef = get_msgdef(typename)
+    msgdef = get_msgdef(typename, typestore)
     func = msgdef.deserialize_cdr_le if little_endian else msgdef.deserialize_cdr_be
-    message, pos = func(rawdata[4:], 0, msgdef.cls)
+    message, pos = func(rawdata[4:], 0, msgdef.cls, typestore)
     assert pos + 4 + 3 >= len(rawdata)
     return message
 
@@ -38,6 +47,7 @@ def serialize_cdr(
     message: object,
     typename: str,
     little_endian: bool = sys.byteorder == 'little',
+    typestore: Typestore = types,
 ) -> memoryview:
     """Serialize message object to bytes.
 
@@ -45,24 +55,25 @@ def serialize_cdr(
         message: Message object.
         typename: Message type name.
         little_endian: Should use little endianess.
+        typestore: Type store.
 
     Returns:
         Serialized bytes.
 
     """
-    msgdef = get_msgdef(typename)
-    size = 4 + msgdef.getsize_cdr(0, message)
+    msgdef = get_msgdef(typename, typestore)
+    size = 4 + msgdef.getsize_cdr(0, message, typestore)
     rawdata = memoryview(bytearray(size))
     pack_into('BB', rawdata, 0, 0, little_endian)
 
     func = msgdef.serialize_cdr_le if little_endian else msgdef.serialize_cdr_be
 
-    pos = func(rawdata[4:], 0, message)
+    pos = func(rawdata[4:], 0, message, typestore)
     assert pos + 4 == size
     return rawdata.toreadonly()
 
 
-def ros1_to_cdr(raw: bytes, typename: str) -> memoryview:
+def ros1_to_cdr(raw: bytes, typename: str, typestore: Typestore = types) -> memoryview:
     """Convert serialized ROS1 message directly to CDR.
 
     This should be reasonably fast as conversions happen on a byte-level
@@ -71,18 +82,20 @@ def ros1_to_cdr(raw: bytes, typename: str) -> memoryview:
     Args:
         raw: ROS1 serialized message.
         typename: Message type name.
+        typestore: Type store.
 
     Returns:
         CDR serialized message.
 
     """
-    msgdef = get_msgdef(typename)
+    msgdef = get_msgdef(typename, typestore)
 
     ipos, opos = msgdef.getsize_ros1_to_cdr(
         raw,
         0,
         None,
         0,
+        typestore,
     )
     assert ipos == len(raw)
 
@@ -96,13 +109,14 @@ def ros1_to_cdr(raw: bytes, typename: str) -> memoryview:
         0,
         rawdata[4:],
         0,
+        typestore,
     )
     assert ipos == len(raw)
     assert opos + 4 == size
     return rawdata.toreadonly()
 
 
-def cdr_to_ros1(raw: bytes, typename: str) -> memoryview:
+def cdr_to_ros1(raw: bytes, typename: str, typestore: Typestore = types) -> memoryview:
     """Convert serialized CDR message directly to ROS1.
 
     This should be reasonably fast as conversions happen on a byte-level
@@ -111,6 +125,7 @@ def cdr_to_ros1(raw: bytes, typename: str) -> memoryview:
     Args:
         raw: CDR serialized message.
         typename: Message type name.
+        typestore: Type store.
 
     Returns:
         ROS1 serialized message.
@@ -118,13 +133,14 @@ def cdr_to_ros1(raw: bytes, typename: str) -> memoryview:
     """
     assert raw[1] == 1, 'Message byte order is not little endian'
 
-    msgdef = get_msgdef(typename)
+    msgdef = get_msgdef(typename, typestore)
 
     ipos, opos = msgdef.getsize_cdr_to_ros1(
         raw[4:],
         0,
         None,
         0,
+        typestore,
     )
     assert ipos + 4 + 3 >= len(raw)
 
@@ -137,6 +153,7 @@ def cdr_to_ros1(raw: bytes, typename: str) -> memoryview:
         0,
         rawdata,
         0,
+        typestore,
     )
     assert ipos + 4 + 3 >= len(raw)
     assert opos == size
diff --git a/src/rosbags/serde/typing.py b/src/rosbags/serde/typing.py
index de4e27b5..5275cc2b 100644
--- a/src/rosbags/serde/typing.py
+++ b/src/rosbags/serde/typing.py
@@ -9,12 +9,14 @@ from typing import TYPE_CHECKING, NamedTuple
 if TYPE_CHECKING:
     from typing import Any, Callable, Tuple
 
-    Bitcvt = Callable[[bytes, int, bytes, int], Tuple[int, int]]
-    BitcvtSize = Callable[[bytes, int, None, int], Tuple[int, int]]
+    from rosbags.typesys.register import Typestore
 
-    CDRDeser = Callable[[bytes, int, type], Tuple[Any, int]]
-    CDRSer = Callable[[bytes, int, object], int]
-    CDRSerSize = Callable[[int, object], int]
+    Bitcvt = Callable[[bytes, int, bytes, int, Typestore], Tuple[int, int]]
+    BitcvtSize = Callable[[bytes, int, None, int, Typestore], Tuple[int, int]]
+
+    CDRDeser = Callable[[bytes, int, type, Typestore], Tuple[Any, int]]
+    CDRSer = Callable[[bytes, int, object, Typestore], int]
+    CDRSerSize = Callable[[int, object, Typestore], int]
 
 
 class Descriptor(NamedTuple):
diff --git a/src/rosbags/typesys/register.py b/src/rosbags/typesys/register.py
index ebbb53bc..21b0714d 100644
--- a/src/rosbags/typesys/register.py
+++ b/src/rosbags/typesys/register.py
@@ -13,10 +13,16 @@ from . import types
 from .base import Nodetype, TypesysError
 
 if TYPE_CHECKING:
-    from typing import Any, Optional, Union
+    from typing import Any, Optional, Protocol, Union
 
     from .base import Typesdict
 
+    class Typestore(Protocol):  # pylint: disable=too-few-public-methods
+        """Type storage."""
+
+        FIELDDEFS: Typesdict
+
+
 INTLIKE = re.compile('^u?(bool|int|float)')
 
 
@@ -134,11 +140,12 @@ def generate_python_code(typs: Typesdict) -> str:
     return '\n'.join(lines)
 
 
-def register_types(typs: Typesdict) -> None:
+def register_types(typs: Typesdict, typestore: Typestore = types) -> None:
     """Register types in type system.
 
     Args:
         typs: Dictionary mapping message typenames to parsetrees.
+        typestore: Type store.
 
     Raises:
         TypesysError: Type already present with different definition.
@@ -156,14 +163,14 @@ def register_types(typs: Typesdict) -> None:
     for name, (_, fields) in fielddefs.items():
         if name == 'std_msgs/msg/Header':
             continue
-        if have := types.FIELDDEFS.get(name):
+        if have := typestore.FIELDDEFS.get(name):
             _, have_fields = have
             have_fields = [(x[0].lower(), x[1]) for x in have_fields]
             fields = [(x[0].lower(), x[1]) for x in fields]
             if have_fields != fields:
                 raise TypesysError(f'Type {name!r} is already present with different definition.')
 
-    for name in fielddefs.keys() - types.FIELDDEFS.keys():
+    for name in fielddefs.keys() - typestore.FIELDDEFS.keys():
         pyname = name.replace('/', '__')
-        setattr(types, pyname, getattr(module, pyname))
-        types.FIELDDEFS[name] = fielddefs[name]
+        setattr(typestore, pyname, getattr(module, pyname))
+        typestore.FIELDDEFS[name] = fielddefs[name]
diff --git a/tests/cdr.py b/tests/cdr.py
index 562329f3..c7dbd76a 100644
--- a/tests/cdr.py
+++ b/tests/cdr.py
@@ -14,6 +14,7 @@ from numpy.typing import NDArray
 from rosbags.serde.messages import SerdeError, get_msgdef
 from rosbags.serde.typing import Msgdef
 from rosbags.serde.utils import SIZEMAP, Valtype
+from rosbags.typesys import types
 
 if TYPE_CHECKING:
     from typing import Any, Tuple
@@ -187,7 +188,7 @@ def deserialize(rawdata: bytes, typename: str) -> Msgdef:
     """
     _, little_endian = unpack_from('BB', rawdata, 0)
 
-    msgdef = get_msgdef(typename)
+    msgdef = get_msgdef(typename, types)
     obj, _ = deserialize_message(
         rawdata[4:],
         BASETYPEMAP_LE if little_endian else BASETYPEMAP_BE,
@@ -428,7 +429,7 @@ def serialize(
         Serialized bytes.
 
     """
-    msgdef = get_msgdef(typename)
+    msgdef = get_msgdef(typename, types)
     size = 4 + get_size(message, msgdef)
     rawdata = memoryview(bytearray(size))
 
diff --git a/tests/test_serde.py b/tests/test_serde.py
index 8359f52f..276bcd6c 100644
--- a/tests/test_serde.py
+++ b/tests/test_serde.py
@@ -12,7 +12,7 @@ import pytest
 
 from rosbags.serde import SerdeError, cdr_to_ros1, deserialize_cdr, ros1_to_cdr, serialize_cdr
 from rosbags.serde.messages import get_msgdef
-from rosbags.typesys import get_types_from_msg, register_types
+from rosbags.typesys import get_types_from_msg, register_types, types
 from rosbags.typesys.types import builtin_interfaces__msg__Time as Time
 from rosbags.typesys.types import geometry_msgs__msg__Polygon as Polygon
 from rosbags.typesys.types import sensor_msgs__msg__MagneticField as MagneticField
@@ -316,14 +316,14 @@ def test_custom_type() -> None:
     register_types(dict(get_types_from_msg(DYNAMIC_S_64, 'test_msgs/msg/dynamic_s_64')))
     register_types(dict(get_types_from_msg(CUSTOM, cname)))
 
-    static_64_64 = get_msgdef('test_msgs/msg/static_64_64').cls
-    static_64_16 = get_msgdef('test_msgs/msg/static_64_16').cls
-    static_16_64 = get_msgdef('test_msgs/msg/static_16_64').cls
-    dynamic_64_64 = get_msgdef('test_msgs/msg/dynamic_64_64').cls
-    dynamic_64_b_64 = get_msgdef('test_msgs/msg/dynamic_64_b_64').cls
-    dynamic_64_s = get_msgdef('test_msgs/msg/dynamic_64_s').cls
-    dynamic_s_64 = get_msgdef('test_msgs/msg/dynamic_s_64').cls
-    custom = get_msgdef('test_msgs/msg/custom').cls
+    static_64_64 = get_msgdef('test_msgs/msg/static_64_64', types).cls
+    static_64_16 = get_msgdef('test_msgs/msg/static_64_16', types).cls
+    static_16_64 = get_msgdef('test_msgs/msg/static_16_64', types).cls
+    dynamic_64_64 = get_msgdef('test_msgs/msg/dynamic_64_64', types).cls
+    dynamic_64_b_64 = get_msgdef('test_msgs/msg/dynamic_64_b_64', types).cls
+    dynamic_64_s = get_msgdef('test_msgs/msg/dynamic_64_s', types).cls
+    dynamic_s_64 = get_msgdef('test_msgs/msg/dynamic_s_64', types).cls
+    custom = get_msgdef('test_msgs/msg/custom', types).cls
 
     msg = custom(
         'str',
@@ -439,7 +439,7 @@ def test_padding_empty_sequence() -> None:
     """Test empty sequences do not add item padding."""
     register_types(dict(get_types_from_msg(SU64_B, 'test_msgs/msg/su64_b')))
 
-    su64_b = get_msgdef('test_msgs/msg/su64_b').cls
+    su64_b = get_msgdef('test_msgs/msg/su64_b', types).cls
     msg = su64_b(numpy.array([], dtype=numpy.uint64), True)
 
     cdr = serialize_cdr(msg, msg.__msgtype__)
@@ -458,7 +458,7 @@ def test_align_after_empty_sequence() -> None:
     """Test alignment after empty sequences."""
     register_types(dict(get_types_from_msg(SU64_U64, 'test_msgs/msg/su64_u64')))
 
-    su64_b = get_msgdef('test_msgs/msg/su64_u64').cls
+    su64_b = get_msgdef('test_msgs/msg/su64_u64', types).cls
     msg = su64_b(numpy.array([], dtype=numpy.uint64), 42)
 
     cdr = serialize_cdr(msg, msg.__msgtype__)