Index: Lib/csv.py
===================================================================
--- Lib/csv.py	(revision 53441)
+++ Lib/csv.py	(working copy)
@@ -115,9 +115,12 @@
 
     def _dict_to_list(self, rowdict):
         if self.extrasaction == "raise":
+            errored_fields = []
             for k in rowdict.keys():
                 if k not in self.fieldnames:
-                    raise ValueError, "dict contains fields not in fieldnames"
+                    errored_fields.append(k)
+            if errored_fields:
+                raise ValueError, "dict contains fields not in fieldnames: %s" % (", ".join(errored_fields))
         return [rowdict.get(key, self.restval) for key in self.fieldnames]
 
     def writerow(self, rowdict):