Peptide toxins from the cyanobacteria Microcystic aeruginosa, Oscillatoria agardhii, Nodularia spumigena, and Anabaena flos‐aquae were isolated. Of these, only the cyclic peptide of Microcystis has been characterized structurally and toxicologically in detail. Preliminary studies of the other peptide toxins have indicated that they may be structurally related to the toxins of Microcystis. The morphological and ultrastructural effects of these toxins were studied using freshly isolated hepatocytes. The toxins caused identical morphological alterations in the hepatocytes. Numerous surface swellings appeared on the cell surface and finally the normal cell morphology was totally disrupted. During this process, however, the viability of the cells was not affected. These results are viewed in the light of recent results showing that these toxins could exhibit a specific interaction with the actin part of the cytoskeleton. The overall results indicate that all four toxins may be toxicologically closely related.