When grown under constant conditions of temperature, salinity, water flow, light and food supply, the clam Tapes philippinarum (Adams & Reeve, 1850) lays down weak micro-growth bands in the shell at approximately semi-diurnal intervals, although with wide variations in periodicity between individuals. Clams continuously immersed under natural light--dark cycles under a raft or below the tide mark produce similar bands. These bands are very weak compared with those in the shells of clams allowed to grow intertidally or under simulated semidiurnal tidal conditions of emersion and immersion. The shell bands in intertidal animals show a good correspondence with the number of emersions. Clams subjected to a diurnal cycle of immersion deposit strong bands coinciding approximately with the number of daily emersions, together with faint bands. Thus the total number of bands exceeds the number of emersions by as much as 30-100% in various individuals. There is no discernible influence of cycles of illumination on the banding patterns under any of the experimental conditions. The clam lays down bands during periodic aerial emersion which are much stronger than those formed under constant immersion and due to the endogenous rhythm alone. The results may be interpreted in terms of an endogenous circa-tidal rhythm of shell deposition which can be entrained by periods of tidal emersion.