diff --git a/tools/run_functional.sh b/tools/run_functional.sh
index 8a0c380a..479df632 100755
--- a/tools/run_functional.sh
+++ b/tools/run_functional.sh
@@ -26,4 +26,4 @@ os_service_type=container
 os_endpoint_type=public
 END
 fi
-tox -e functional
+tox -e functional -- --concurrency=1