Malathion (O,O-dimethyl-S-1, 2-di-(ethoxycarbamyl)ethyl phosphorodithioate) is an organophosphate which is a widely used pesticide in the United States. This pesticide seems to be more toxic to insects and fish than to mammals due to the lack of hydrolytic enzymes in insects and fish. Malathion clearly affects the hematology of fish. The study reported here was designed to determine the effects of a commercially available malathion solution on channel catfish Ictalurus punctatus. Objectives were to measure hematological changes and histopathological effects of a sublethal concentration of malathion on the fish.