1. Vertical and horizontal movement were studied in the freshwater bivalve Elliptio complanata at a sandy site in an oligotrophic lake over 3 years. Mussel movement did not vary significantly between day and night. On average, between 2 and 8% of 527 mussels moved each month during the ice-free season and the distance travelled by moving mussels averaged 0.6 cm day-1. 2. Mussels were endobenthic during the winter, emerged from the sandy substrate in mid-May, peaked in sediment surface abundance in July, and descended into the sediments for the winter in September-October. Vertical displacement of mussels was closely correlated with water temperature although daylength may play a role. Mussels apparently move very little beneath the sediment during the winter. 3. The number of mussels moving horizontally at any given time was linearly correlated with daylength, but the distance travelled during a sampling period was related to daylength in a non-linear fashion. Greatest horizontal displacement of epibenthic mussels was found during spring and early summer, coincident with spawning in E. complanata.