Raphidophyceae have been recorded yearly in north-western Europe since 1989; the number of observations has increased steadily. At least 4 species have been recognised in Dutch coastal waters: Chattonella antiqua, Chattonella marina, Fibrocapsa japonica and Heterosigma akashiwo. Occasionally small blooms of Raphidophytes have been recorded in this region, most recently in July/August of 1998 when large numbers ( > 10⁴ cells/l) of Fibrocapsa japonica were observed.Raphidophyte species are known to produce toxins, whereas unfavourable environmental conditions induce cyst formation. So far, cysts of Raphidophytes have not been reported in the southern North Sea. For most European strains of Raphidophytes toxin synthesis have not been analysed. Preliminary studies on Fibrocapsa japonica, grown under different ratios of nitrate and phosphate, showed that both at low nitrate and phosphate concentrations had a short period of growth. Toxin concentrations appeared to be highest when growth ceased especially after phosphate became limiting. The toxin profiles changed during the age of the cultures, while most components diminished when cultures declined. Toxicity tests on the individually sampled compounds are yet to be performed. For further research it is very important to determine the set of environmental conditions that favours bloom formation. Complementary field studies will be conducted to test the relevance of our laboratory experiments.