The annual reproductive cycle of Anomia ephippium (L.) in Strangford Lough, N. Ireland was investigated from histological preparations of the gonad. Development proceeded rapidly during late spring until by June most individuals were fully ripe. Spawning occurred over the summer months (June—August) after which the reproductive follicles gradually regressed and the population entered a resting or spent phase. Changes in gross shell morphology occurred with increased body size. Shells became progressively more ovoid in shape with proportionately smaller byssal embayments in the lower valve. Differences in the relative rates of growth of the upper and lower shell valves are probably explicable in terms of their relative mechanical efficiencies. Shell growth was maximal during late spring and early summer.