In the late 1980s, recruitment failures of the mussel Mytilus edulis led to economic problems in the mussel fishing and cultivation industries of northwestern Europe. As part of a collaborative study to gain a better understanding of the mechanisms affecting recruitment processes of mussels, plankton samples were collected regularly over a four-year period (t990-I993) from three stations in the Schleswig-Holstein Wadden Sea. The bivalve component of the plankton was dominated by the Solenidae, which was almost exclusively represented by Ensis americanus [= directus). NI. eduhs was the second most abundant species. Abundances of mussel larvae peaked 2 to 4 weeks after spawning maxima in the adult populations. Although variations in timing and amplitude of the tota~ [arvaL densities occurred, annua[ abuadances o[M. edulis larvae remained stable during the study period, and regional abundance differences were insignificant. A close relaUonsh~p was found between peaks in larval abundance and phytoplankton blooms. Differences in larval concentrations in the ebb and the flow currents were insignificant. Planktic mussel larvae measured between 200 }am and 300 ~tm, and successive cohorts were recognizable in the majority of samples. Most larvae were found to originate from local stocks, although imports from outside the area do occur.