diff --git a/src/rosbags/interfaces/__init__.py b/src/rosbags/interfaces/__init__.py
index df5b221c..c60e1ea0 100644
--- a/src/rosbags/interfaces/__init__.py
+++ b/src/rosbags/interfaces/__init__.py
@@ -34,3 +34,12 @@ class Connection(NamedTuple):
     md5sum: str
     msgcount: int
     ext: Union[ConnectionExtRosbag1, ConnectionExtRosbag2]
+
+
+class TopicInfo(NamedTuple):
+    """Topic information."""
+
+    msgtype: Optional[str]
+    msgdef: Optional[str]
+    msgcount: int
+    connections: list[Connection]
diff --git a/src/rosbags/rosbag1/reader.py b/src/rosbags/rosbag1/reader.py
index 99ec9014..4034ff1b 100644
--- a/src/rosbags/rosbag1/reader.py
+++ b/src/rosbags/rosbag1/reader.py
@@ -20,7 +20,7 @@ from typing import TYPE_CHECKING, Any, Dict, NamedTuple
 
 from lz4.frame import decompress as lz4_decompress
 
-from rosbags.interfaces import Connection, ConnectionExtRosbag1
+from rosbags.interfaces import Connection, ConnectionExtRosbag1, TopicInfo
 from rosbags.typesys.msg import normalize_msgtype
 
 if TYPE_CHECKING:
@@ -68,15 +68,6 @@ class Chunk(NamedTuple):
     decompressor: Callable[[bytes], bytes]
 
 
-class TopicInfo(NamedTuple):
-    """Topic information."""
-
-    conncount: int
-    msgcount: int
-    msgdef: str
-    msgtype: str
-
-
 class IndexData(NamedTuple):
     """Index data."""
 
@@ -336,8 +327,6 @@ class Reader:
 
     """
 
-    # pylint: disable=too-many-instance-attributes
-
     def __init__(self, path: Union[str, Path]):
         """Initialize.
 
@@ -358,7 +347,6 @@ class Reader:
         self.chunk_infos: list[ChunkInfo] = []
         self.chunks: dict[int, Chunk] = {}
         self.current_chunk: tuple[int, BinaryIO] = (-1, BytesIO())
-        self.topics: dict[str, TopicInfo] = {}
 
     def open(self) -> None:  # pylint: disable=too-many-locals
         """Open rosbag and read metadata."""
@@ -422,28 +410,6 @@ class Reader:
                     len(self.indexes[cid]),
                     connection[6],
                 )
-
-            self.topics = {}
-            for topic, group in groupby(
-                sorted(self.connections.values(), key=lambda x: x.topic),
-                key=lambda x: x.topic,
-            ):
-                connections = list(group)
-                count = reduce(
-                    lambda x, y: x + y,
-                    (
-                        y.connection_counts.get(x.id, 0)
-                        for x in connections
-                        for y in self.chunk_infos
-                    ),
-                )
-
-                self.topics[topic] = TopicInfo(
-                    len(connections),
-                    count,
-                    connections[0].msgdef,
-                    connections[0].msgtype,
-                )
         except ReaderError:
             self.close()
             raise
@@ -474,6 +440,28 @@ class Reader:
         """Total message count."""
         return reduce(lambda x, y: x + y, (x.msgcount for x in self.topics.values()), 0)
 
+    @property
+    def topics(self) -> dict[str, TopicInfo]:
+        """Topic information."""
+        topics = {}
+        for topic, group in groupby(
+            sorted(self.connections.values(), key=lambda x: x.topic),
+            key=lambda x: x.topic,
+        ):
+            connections = list(group)
+            msgcount = reduce(
+                lambda x, y: x + y,
+                (y.connection_counts.get(x.id, 0) for x in connections for y in self.chunk_infos),
+            )
+
+            topics[topic] = TopicInfo(
+                msgtypes.pop() if len(msgtypes := {x.msgtype for x in connections}) == 1 else None,
+                msgdefs.pop() if len(msgdefs := {x.msgdef for x in connections}) == 1 else None,
+                msgcount,
+                connections,
+            )
+        return topics
+
     def read_connection(self) -> tuple[int, Connection]:
         """Read connection record from current position."""
         assert self.bio
diff --git a/src/rosbags/rosbag2/reader.py b/src/rosbags/rosbag2/reader.py
index 72ea005d..c9ec5082 100644
--- a/src/rosbags/rosbag2/reader.py
+++ b/src/rosbags/rosbag2/reader.py
@@ -14,7 +14,7 @@ import zstandard
 from ruamel.yaml import YAML
 from ruamel.yaml.error import YAMLError
 
-from rosbags.interfaces import Connection, ConnectionExtRosbag2
+from rosbags.interfaces import Connection, ConnectionExtRosbag2, TopicInfo
 
 if TYPE_CHECKING:
     from types import TracebackType
@@ -206,13 +206,12 @@ class Reader:
         return mode if mode != 'none' else None
 
     @property
-    def topics(self) -> dict[str, Connection]:
-        """Topic information.
-
-        For the moment this a dictionary mapping topic names to connections.
-
-        """
-        return {x.topic: x for x in self.connections.values()}
+    def topics(self) -> dict[str, TopicInfo]:
+        """Topic information."""
+        return {
+            x.topic: TopicInfo(x.msgtype, x.msgdef, x.msgcount, [x])
+            for x in self.connections.values()
+        }
 
     def messages(  # pylint: disable=too-many-locals
         self,