diff --git a/csp/middleware.py b/csp/middleware.py
index cb13333..f85219e 100644
--- a/csp/middleware.py
+++ b/csp/middleware.py
@@ -70,7 +70,9 @@ def process_response(self, request, response):
                 headers[header].append(csp)
 
         for header, policies in headers.items():
-            response[header] = '; '.join(policies)
+            # Multiple policies are joined by a comma and should be treated by
+            # the browser as though they were delivered under multiple headers.
+            response[header] = ', '.join(policies)
         return response
 
     def build_policy(self, request, response):