Alpha-linolenic acid and linoleic acid isolated from Ulva fasciata showed toxic effects on red tide phytoplankters in a concentration-dependent manner. Among six species tested, raphidophycean flagellate Heterosigma akashiwo was the most susceptible to these fatty acids, and 50% lethal concentrations (LC50) of alpha-linolenic acid and linoleic acid were estimated to be 0.58 and 1.91 microg/ml respectively, whereas dinoflagellate Gymnodinium impudicum and Heterocapsa circularisquama were highly resistant and no significant toxic effects were observed up to 1,000 microg/ml. Both fatty acids were less toxic to fish (devil stinger), zooplankters (brine shrimp and rotifer), and mammalian cell lines (U937, HeLa, Vero, and CHO cells) than H. akashiwo.