--- src/twisted/plugin.py
+++ src/twisted/plugin.py
@@ -180,7 +180,7 @@
             if pluginKey not in existingKeys:
                 del dropinDotCache[pluginKey]
                 needsWrite = True
-        if needsWrite:
+        if needsWrite and os.environ.get("TWISTED_DISABLE_WRITING_OF_PLUGIN_CACHE") is None:
             try:
                 dropinPath.setContent(pickle.dumps(dropinDotCache))
             except: