By sampling with special apparatus from a ship it was possible to get an idea of the molluscs on the riverbed of four different areas (fig. 1) in the western part of the Netherlands. The species that were found are presented in Table 1 (living animals) and Table 2 (empty shells). Corbicula fluminalis is now ubiqutious in the Netherlands and also in our samples. The samples of the river Nieuwe Maas and one of the harbours (2e Petroleumhaven) in Rotterdam are extremely poor, probably due to the poor quality of the environment and the changing salinity. Notable are the different species not indigenous in the Netherlands (and Europe), such as the two Corbicula ’s, Dreissena, Lithoglyphus, Menetes, Mytilopsis, Physella and Potamopyrgus.