Various axenic strains of Microcystis aeruginosa were found to have different short term toxic effects on Daphnia magna. One of these toxic manifestations, the "blocking" effect, markedly reduced the food uptake by the daphnids. In addition, several of the Microcystis strains are lethal for juvenile and adult daphnids. No correlations were found between the blocking of ingestion, lethality to daphnids, and the "mouse-killing" factors of Microcystis. Thus, several toxic principles are responsible for the different short term toxic manifestations.Microcystis aeruginosa is commonly involved in freshwater blooms. On many occasions such blooms have been followed by a mass poisoning of livestock and wild life (mammals and birds). These toxigenic blooms have been reported from many countries (Schwimmer and Schwimmer 1969; Carmichael 198 1;Codd 1984). Studies of Microcystis toxins have shown that more than a single toxic principle is involved (Foxall and Sasner 198 l), with different toxic manifestations (Codd and Carmichael 1982;Slatkin et al. 1983), different species specificities (Foxall and Sasner 198 l), various molecular weights (Runnegar and Falconer 198 l), and different amino acid composition (Santikarn et al. 1983).Increasing attention has been given to the relationship between M. aeruginosa and aquatic grazers such as Daphnia magna (Lampert 198 1, 1982