diff --git a/test_tracetools/test/test_generic_subscription.py b/test_tracetools/test/test_generic_subscription.py
index 8e4545a2..c0241da7 100644
--- a/test_tracetools/test/test_generic_subscription.py
+++ b/test_tracetools/test/test_generic_subscription.py
@@ -66,7 +66,7 @@ def test_all(self):
 
         for event in rmw_sub_init_events:
             self.assertValidHandle(event, ['rmw_subscription_handle'])
-            self.assertValidStaticArray(event, 'gid', int, 24)
+            self.assertValidStaticArray(event, 'gid', int, 16)
         for event in rcl_sub_init_events:
             self.assertValidHandle(
                 event,
diff --git a/test_tracetools/test/test_publisher.py b/test_tracetools/test/test_publisher.py
index 4d9e1666..c3ce86e8 100644
--- a/test_tracetools/test/test_publisher.py
+++ b/test_tracetools/test/test_publisher.py
@@ -47,7 +47,7 @@ def test_all(self):
         rmw_pub_init_events = self.get_events_with_name(tp.rmw_publisher_init)
         for event in rmw_pub_init_events:
             self.assertValidHandle(event, ['rmw_publisher_handle'])
-            self.assertValidStaticArray(event, 'gid', int, 24)
+            self.assertValidStaticArray(event, 'gid', int, 16)
         pub_init_events = self.get_events_with_name(tp.rcl_publisher_init)
         for event in pub_init_events:
             self.assertValidHandle(
diff --git a/test_tracetools/test/test_subscription.py b/test_tracetools/test/test_subscription.py
index 77dd6c07..483e4bf0 100644
--- a/test_tracetools/test/test_subscription.py
+++ b/test_tracetools/test/test_subscription.py
@@ -65,7 +65,7 @@ def test_all(self):
 
         for event in rmw_sub_init_events:
             self.assertValidHandle(event, ['rmw_subscription_handle'])
-            self.assertValidStaticArray(event, 'gid', int, 24)
+            self.assertValidStaticArray(event, 'gid', int, 16)
         for event in rcl_sub_init_events:
             self.assertValidHandle(
                 event,
diff --git a/tracetools/include/tracetools/tp_call.h b/tracetools/include/tracetools/tp_call.h
index c56dc1b4..0b63de14 100644
--- a/tracetools/include/tracetools/tp_call.h
+++ b/tracetools/include/tracetools/tp_call.h
@@ -35,7 +35,7 @@
 #include "tracetools/version.h"
 
 /// See RMW_GID_STORAGE_SIZE in rmw.
-#define TRACETOOLS_GID_STORAGE_SIZE 24u
+#define TRACETOOLS_GID_STORAGE_SIZE 16u
 
 #ifdef __clang__
 # pragma clang diagnostic push