diff --git a/google/cloud/pubsublite/cloudpubsub/internal/multiplexed_subscriber_client.py b/google/cloud/pubsublite/cloudpubsub/internal/multiplexed_subscriber_client.py
index 928984ae..3c4d9dfe 100644
--- a/google/cloud/pubsublite/cloudpubsub/internal/multiplexed_subscriber_client.py
+++ b/google/cloud/pubsublite/cloudpubsub/internal/multiplexed_subscriber_client.py
@@ -13,7 +13,7 @@
 # limitations under the License.
 
 from concurrent.futures.thread import ThreadPoolExecutor
-from typing import Union, Optional, Set
+from typing import Union, Optional, Set, Callable, cast
 
 from google.cloud.pubsub_v1.subscriber.futures import StreamingPullFuture
 
@@ -61,12 +61,13 @@ def cancel_streaming_pull_future(fut: StreamingPullFuture):
     def subscribe(
         self,
         subscription: Union[SubscriptionPath, str],
-        callback: MessageCallback,
+        callback: Callable,  # TODO(dpcollins): Change to MessageCallback,
         per_partition_flow_control_settings: FlowControlSettings,
         fixed_partitions: Optional[Set[Partition]] = None,
     ) -> StreamingPullFuture:
         if isinstance(subscription, str):
             subscription = SubscriptionPath.parse(subscription)
+        callback = cast(MessageCallback, callback)
 
         def create_and_open():
             underlying = self._underlying_factory(
diff --git a/google/cloud/pubsublite/cloudpubsub/subscriber_client.py b/google/cloud/pubsublite/cloudpubsub/subscriber_client.py
index 13705beb..12ecdae5 100644
--- a/google/cloud/pubsublite/cloudpubsub/subscriber_client.py
+++ b/google/cloud/pubsublite/cloudpubsub/subscriber_client.py
@@ -13,7 +13,7 @@
 # limitations under the License.
 
 from concurrent.futures.thread import ThreadPoolExecutor
-from typing import Optional, Union, Set, AsyncIterator
+from typing import Optional, Union, Set, AsyncIterator, Callable
 
 from google.api_core.client_options import ClientOptions
 from google.auth.credentials import Credentials
@@ -33,7 +33,6 @@
 from google.cloud.pubsublite.cloudpubsub.nack_handler import NackHandler
 from google.cloud.pubsublite.cloudpubsub.subscriber_client_interface import (
     SubscriberClientInterface,
-    MessageCallback,
     AsyncSubscriberClientInterface,
 )
 from google.cloud.pubsublite.internal.constructable_from_service_account import (
@@ -100,7 +99,7 @@ def __init__(
     def subscribe(
         self,
         subscription: Union[SubscriptionPath, str],
-        callback: MessageCallback,
+        callback: Callable,  # TODO(dpcollins): Change to MessageCallback,
         per_partition_flow_control_settings: FlowControlSettings,
         fixed_partitions: Optional[Set[Partition]] = None,
     ) -> StreamingPullFuture:
diff --git a/google/cloud/pubsublite/cloudpubsub/subscriber_client_interface.py b/google/cloud/pubsublite/cloudpubsub/subscriber_client_interface.py
index 8dd23506..e9e43113 100644
--- a/google/cloud/pubsublite/cloudpubsub/subscriber_client_interface.py
+++ b/google/cloud/pubsublite/cloudpubsub/subscriber_client_interface.py
@@ -81,7 +81,7 @@ class SubscriberClientInterface(ContextManager):
     def subscribe(
         self,
         subscription: Union[SubscriptionPath, str],
-        callback: MessageCallback,
+        callback: Callable,  # TODO(dpcollins): Change to MessageCallback,
         per_partition_flow_control_settings: FlowControlSettings,
         fixed_partitions: Optional[Set[Partition]] = None,
     ) -> StreamingPullFuture: