Numerous species of fish secrete from their skin a class of toxins called icthyocrinotoxins, which protect them from being eaten by larger fish including sharks. The lethality of these toxins has been studied in small fish, and their effect on sharks has been documented by tonic immobility tests and by the ability to induce food refusal. The icthyocrinotoxins which have been adequately studied represent a variety of chemical structures, which have the common characteristic of being surfactants. They exhibit various properties characteristic of surfactants, including the ability to disrupt biological membranes, hemolysis and icthyotoxicity to small fish. The largest group of icthyocrinotoxins are polypeptide surfactants. The best studied are the paradaxins from the moses sole (Pardachirus marmoratus), which are polypeptides (M.W. -13, 000) with some structural similarity to melittin, the well-studied bee venom peptide. The next largest group are steroid glycosides, including the pavoninins and mosesins, which contain a cholestane skeleton with a variety of monosaccharides attached to either the B or D ring. The third type are cationic surfactants typified by pahutoxin from the boxfish, Ostracicon lentiginosus, which is a fatty acid choline ester. Other surfactants, including synthetics such as sodium dodecysulfate, exhibit potent activity in tests used for these types of toxins. Nevertheless they appear to have little potential as shark repellents due to the difficulty in maintaining high localized concentrations for extended periods.