The distribution, age structure, and movements of the unionid musselElliptio complanata(Lightfoot) were studied in a first-order stream in Virginia, USA. Mean density of the mussel in this low-gradient, sand-bottomed stream was 2.5 individuals/m2and biomass was 3.4 g dry mass/m2. About 87% of the population had a shell length of 6–9 cm, or age of 4–6 years. Only 11% of the population was less than four years old; the age of the oldest individuals encountered was only eight years. Distribution of the mussels was highly clumped, but no physical, chemical, or hydrologic factors examined were significantly correlated with mussel abundance. All young mussels (age < three years) were burrowed into the sediment, whereas older individuals occurred both below and at the sediment surface, depending on time of year. About 90% of the population was burrowed below the surface during the winter, but a high of 80% of the mussels moved to the surface in March and April, the time of peak reproductive activity. Tagged mussels moved an average of 2.9 m during one year. Although the direction of those movements was erratic, the overall movement of the population was a net 27 cm downstream, indicating no directed upstream movement to compensate for downstream displacement during storms.