A floristic survey of the benthic diatom flora of brackish Lake Grevelingen, a former estuary in the S.W. Netherlands, revealed the presence of 189 pennate taxa (42 genera) and 31 centrical taxa (18 genera). Information on habitat, presence and abundance of the diatom species has been added. Of these 220 taxa 21 species are reported as new for the Netherlands' diatom flora. The diatom assemblage of Lake Grevelingen differs considerably from that of part of the Oosterschelde and of the Wadden Sea.