The toxicity of the marine phytoflagellates Prymnesium parvum. Prymnesium patelliferum, Chrysochromulina polylepis, and Chrysochromulina leadbeateri isolated from ichthyotoxic blooms in Norwegian coastal waters was compared using four different test methods developed for the detection of toxins produced by these species. The test methods were (1) lethality to the crustacean Artemia salina exposed to living algae, (2) hemolytic activity (lysis of human erythrocytes) by crude algal lipid extracts, and inhibition of the uptake of the neurotransmitters L-glutamate and gamma-aminobutyric acid (GABA) into (3) synaptosomes and (4) synaptic vesicles of rat brain by crude algal lipid extracts. All test methods indicated different levels of toxicity among the algal species. Prymnesium parvum, P. patelliferum, and C. polylepis were toxic as determined by all four test methods. The cultured strain of C. leadbeateri, although isolated from a toxic algal bloom, appeared nontoxic by the methods used here, and served as a negative control. The hemolytic activity of P. parvum extract was about nine times higher than that of P. patelliferum extract, whereas the activity was only two to three times higher using the other three methods. Chrysochromulina polylepis had higher toxic activity than P. patelliferum except for less inhibitory effect on synaptosomes. The inhibition of synaptosomal and vesicular neurotransmitter uptake systems by extracts of P. parvum, P. patelliferum, and C. polylepis appeared to be due to similar mechanisms of action, since similar inhibition ratios between the uptake of L-glutamate and GABA were obtained in both synaptosomes and synaptic vesicles. We suggest that P. parvum, P. patelliferum, and C. polylepis produce a set of similar toxins and that the relative amounts of each toxin vary among the three species.