diff --git a/test_data/of13/group_mod.data b/test_data/of13/group_mod.data
new file mode 100644
index 0000000..8f25b57
--- /dev/null
+++ b/test_data/of13/group_mod.data
@@ -0,0 +1,59 @@
+-- binary
+04 0f # version, type
+00 70 # length
+12 34 56 78 # xid
+00 01 # command
+03 # group_type
+00 # pad
+00 00 00 05 # group_id
+00 30 # buckets[0].len
+00 01 # buckets[0].weight
+00 00 00 05 # buckets[0].watch_port
+ff ff ff ff # buckets[0].watch_group
+00 00 00 00 # pad
+00 00 # buckets[0].actions[0].type
+00 10 # buckets[0].actions[0].len
+00 00 00 05 # buckets[0].actions[0].port
+00 00 # buckets[0].actions[0].max_len
+00 00 00 00 00 00 # pad
+00 00 # buckets[0].actions[1].type
+00 10 # buckets[0].actions[1].len
+00 00 00 06 # buckets[0].actions[1].port
+00 00 # buckets[0].actions[1].max_len
+00 00 00 00 00 00 # pad
+00 30 # buckets[1].len
+00 01 # buckets[1].weight
+00 00 00 06 # buckets[1].watch_port
+ff ff ff ff # buckets[1].watch_group
+00 00 00 00 # pad
+00 00 # buckets[1].actions[0].type
+00 10 # buckets[1].actions[0].len
+00 00 00 05 # buckets[1].actions[0].port
+00 00 # buckets[1].actions[0].max_len
+00 00 00 00 00 00 # pad
+00 00 # buckets[1].actions[1].type
+00 10 # buckets[1].actions[1].len
+00 00 00 06 # buckets[1].actions[1].port
+00 00 # buckets[1].actions[1].max_len
+00 00 00 00 00 00 # pad
+-- python
+ofp.message.group_mod(
+    xid=0x12345678,
+    command=ofp.OFPGC_MODIFY,
+    group_type=ofp.OFPGT_FF,
+    group_id=5,
+    buckets=[
+        ofp.bucket(
+            weight=1,
+            watch_port=5,
+            watch_group=0xffffffff,
+            actions=[
+                ofp.action.output(port=5, max_len=0),
+                ofp.action.output(port=6, max_len=0)]),
+        ofp.bucket(
+            weight=1,
+            watch_port=6,
+            watch_group=0xffffffff,
+            actions=[
+                ofp.action.output(port=5, max_len=0),
+                ofp.action.output(port=6, max_len=0)])])