Colonization of two species of native gastropods,Campeloma crassulum(Viviparidae) andPleurocera canaliculatum(Pleuroceratidae), by the exotic zebra mussel (Dreissena polymorpha) was documented for the Mississippi River and Illinois River in Illinois. Wind- and boat-generated waves created windrowed deposits of these snails. Nearly all snails collected retained intact zebra mussels or evidence of mussel attachment.