Swan Lake, a backwater of the Illinois River, had a smaller population of zebra mussels (Dreissena polymorpha) in 1994 compared with 1993. This reduction in mussel abundance may reflect the controlling influence of lake physical properties and interactions between waterfowl, freshwater drum, andD. polymorpha.Because this lake is inhabited by a diverse unionid fauna (at least 16 species) but by relatively fewD. polymorpha, it could function as a refugia for native mussels from the adverse effects ofD. polymorpha.Other such refugia should be sought and protected as a means of maintaining unionid diversity in the Illinois River and other rivers colonized byD. polymorpha.