After initial spatfall, some species of bivalves and juvenile mud snails may re-enter the water column and thus undergo secondary dispersal. Using continuous plankton net records of the number of drifting organisms over a period of 4 mo in summer 1991, rhythms of migratory activity were analysed on a tidal flat area near the Island of Sylt, eastern North Sea. All abundant species showed a clear diurnal pattern, The mud snail Hydrobia ulvae was more abundant in the water column during daytime than in the dark, while the reverse was true for the bivalves observed. In addition, cockles Cerastoderma edule and razor clams Ensis directus displayed a semilunar rhythm of drifting activity, and in clams Macoma balthica there was a 10 d periodicity not closely linked to lunar phases. In most species, the quantitative effect of these longer periodicities was stronger than that of the diurnal component. The pattern of migratory activity, resulting from these diurnal and lunar rhythms together with a seasonal component, was distorted by weather conditions in a species-specific way. As a consequence, records of drifting young molluscs may be severely biased unless these rhythms of migratory activity are taken into account.