In spring 1993 phytoplankton samples were taken along the longitudinal salinity gradient of the estuaries of the Elbe, Schelde and Gironde. A minimum in phytoplankton abundance, biomass and diversity was observed in the inner to upper reaches of all estuaries, suggesting massive mortality in these zones. Due to a combination of high turbidity and high river runoff, phytoplankton of the Gironde estuary differed in species composition and had lower cell abundancies and biomass compared with Elbe and Schelde. Phytoplankton structure and biomass of the polyhaline reaches of the Elbe and Schelde were quite comparable. They were characterized by the diatoms Skeletonema costatum and several Thalassiosira spp. and most probably developed in situ. The presence of tychoplanktonic species in these reaches indicates exchange with the intertidal flats. On the other hand, some marked differences were observed in the upper and tidal freshwater reaches of these two estuaries. In the oligohaline Schelde stations, phytoplankton populations were dominated by the diatom Cyclotella meneghiniana. The diatom Stephanodiscus hantzschii was dominant in the limnetic stations of the Elbe estuary. Despite the unfavourable light climate, the presence of the halophilous diatom C. meneghiniana in the Schelde estuary might indicate in situ development of phytoplankton populations. This could be attributed to the high residence time and the absence of grazers due to oxygen stress. In the Elbe, river runoff is too high (at least in spring) to permit in situ development of phytoplankton in the upper and freshwater tidal reaches.