diff --git a/core/drupalci.yml b/core/drupalci.yml
index ed02d43c6375d58c810d8626bcb16cb0babfd74c..9ce9b2b1c074230e930dce14f29580be0b616ce3 100644
--- a/core/drupalci.yml
+++ b/core/drupalci.yml
@@ -35,6 +35,8 @@ build:
          suppress-deprecations: false
          halt-on-fail: false
       run_tests.build:
+        # Limit concurrency due to disk space concerns.
+        concurrency: 15
         types: 'PHPUnit-Build'
         testgroups: '--all'
         suppress-deprecations: false