Biological monitoring in the Dutch Wadden Sea between November 1993 and July 1994 revealed Pseudo-nitzschia-like pennate chain-forming diatom species with cell numbers ranging from 10^2 to 10^5 per litre. Cultured isolates and field samples were examined by electron microscopy, which revealed the majority to be Pseudo-nitzschia pungens. This species dominated over other diatoms in the phytoplankton population during November 1993 and at the end of June 1994. At the beginning of June 1994, P. fraldulenta was also present; occasionally, P. delicatissima was observed. One isolate showed the characteristic morphology of P. multiseries. Species-specific polyclonal antibodies and large-subunit (LSU) rRNA-targeted oligonucleotides for North American strains of P. multiseries and P. pungens applied to the European isolates, confirmed species designations based on electron microscopy. The isolate of P. multiseries from the Dutch Wadden Sea produced domoic acid; after 55 days of growth about 19 pg per cell was measured. This is the first report of a domoic acid-producing P. multiseries isolated from European coastal waters.