Since the massive bloom in 1988 in the North Sea, the prymnesiophyte flagellate Chrysochromulina polylepis Manton et Parke has been known for its ichtyotoxicity. Laboratory experiments using two clones of C. polylepis were conducted in a comparative approach. The clones were similar in size and shape, but differed in their toxicity, as demonstrated by the Artemia bioassay. To study the effects of toxic C. polylepis on protozooplankton grazers, grazing experiments were performed with the heterotrophic dinoflagellate Oxyrrhis marina Dujardin as grazer. A first experiment was carried out in order to follow batch culture growth and initial grazing of O. marina when fed toxic or non-toxic clones of C. polylepis. Ingestion of the toxic clone was 27 % of ingestion when fed with the non-toxic clone. When O. marina was fed with the toxic clone, vacuoles within O. marina cells contained fewer food particles and the cells grew slower (58 % of the division rate estimated for the non-toxic clone). A second experiment was conducted to determine the grazing and growth response of O. marina as a function of algal food concentration. Profound differences in ingestion, clearance, division, and gross growth efficiency of O. marina when fed the two clones of C. polylepis were again apparent. the toxin acts as a grazers deterrent, even at algal concentrations of 400 x 103 ml-1, O. marina was not killed by the presence or ingestion of toxic C. polylepis. In addition to grazing experiments, lipid classes and fatty acids of both algal clones were analysed and compared in order to follow the hypothesis that toxicity of C. polylepis is caused by liposaccharides, lipids, or fatty acids. However, the chemical composition with respect to lipid classes and fatty acids of both clones were quite similar, making it unlikely that these substances are involved in the toxicity towards Artemia and O. marina.