diff --git a/mypy_protobuf/main.py b/mypy_protobuf/main.py
index 47fe3e7e..78a3cde3 100644
--- a/mypy_protobuf/main.py
+++ b/mypy_protobuf/main.py
@@ -781,6 +781,7 @@ def write_grpc_services(
         self,
         services: Iterable[d.ServiceDescriptorProto],
         scl_prefix: SourceCodeLocation,
+        async_only: bool,
     ) -> None:
         wl = self._write_line
         for i, service in enumerate(services):
@@ -797,21 +798,25 @@ def write_grpc_services(
             with self._indent():
                 if self._write_comments(scl):
                     wl("")
-                # To support casting into FooAsyncStub, allow both Channel and aio.Channel here.
-                channel = f"{self._import('typing', 'Union')}[{self._import('grpc', 'Channel')}, {self._import('grpc.aio', 'Channel')}]"
+                # To support casting into FooAsyncStub, allow both Channel and aio.Channel here,
+                # but only if we are generating both sync and async stubs.
+                channel = self._import("grpc.aio", "Channel")
+                if not async_only:
+                    channel = f"{self._import('typing', 'Union')}[{self._import('grpc', 'Channel')}, {channel}]"
                 wl("def __init__(self, channel: {}) -> None: ...", channel)
-                self.write_grpc_stub_methods(service, scl)
+                self.write_grpc_stub_methods(service, scl, is_async=async_only)
 
-            # The (fake) async stub client
-            wl(
-                "class {}AsyncStub:",
-                service.name,
-            )
-            with self._indent():
-                if self._write_comments(scl):
-                    wl("")
-                # No __init__ since this isn't a real class (yet), and requires manual casting to work.
-                self.write_grpc_stub_methods(service, scl, is_async=True)
+            if not async_only:
+                # The (fake) async stub client
+                wl(
+                    "class {}AsyncStub:",
+                    service.name,
+                )
+                with self._indent():
+                    if self._write_comments(scl):
+                        wl("")
+                    # No __init__ since this isn't a real class (yet), and requires manual casting to work.
+                    self.write_grpc_stub_methods(service, scl, is_async=True)
 
             # The service definition interface
             wl(
@@ -1009,6 +1014,7 @@ def generate_mypy_stubs(
 def generate_mypy_grpc_stubs(
     descriptors: Descriptors,
     response: plugin_pb2.CodeGeneratorResponse,
+    async_only: bool,
     quiet: bool,
     readable_stubs: bool,
     relax_strict_optional_primitives: bool,
@@ -1022,7 +1028,7 @@ def generate_mypy_grpc_stubs(
             grpc=True,
         )
         pkg_writer.write_grpc_async_hacks()
-        pkg_writer.write_grpc_services(fd.service, [d.FileDescriptorProto.SERVICE_FIELD_NUMBER])
+        pkg_writer.write_grpc_services(fd.service, [d.FileDescriptorProto.SERVICE_FIELD_NUMBER], async_only)
 
         assert name == fd.name
         assert fd.name.endswith(".proto")
@@ -1079,6 +1085,7 @@ def grpc() -> None:
         generate_mypy_grpc_stubs(
             Descriptors(request),
             response,
+            "async_only" in request.parameter,
             "quiet" in request.parameter,
             "readable_stubs" in request.parameter,
             "relax_strict_optional_primitives" in request.parameter,