: Endosulfan is an organochlorine insecticide, consisting of a mixture of two isomers: alpha and beta endosulfan. Endosulfan residues were determined in livers and gills of carp exposed to lethal and sublethal concentrations of endosulfan. The fish which were exposed to a lethal concentration contained the highest residue level in both liver and gills. In carp liver, the percentage of beta endosulfan in the residue decreased with time between exposure and collection of samples whereas the percentage of endosulfan sulphate increased. Carp killed by exposure to endosulfan had a significantly greater ratio of beta to alpha endosulfan and a significantly greater percentage of beta endosulfan in their livers. There was no such clear relationship for the residue composition in fish gills. The determination of residue composition, in particular the percentage of beta endosulfan or the ratio of beta to alpha isomers is recommended in investigations of fish kills when endosulfan is a suspected cause.