In the fall of 1992 at an ice-disturbed freshwater site along the St. Lawrence estuary, we found zebra mussels (Dreissena polymorpha) on rocks exposed at low tide. The population size structure suggested that the majority of these mussels had settled in early spring of 1992. In May 1993, we revisited the site and found adult zebra mussels in the cracks and crevices of boulders exposed at low tide, but there was a significant reduction in their numbers. Zebra mussels now appear able to invade the intertidal region and to survive the perils of exposure and desiccation in the summer as well as the abrasive action of ice scour in the winter.