The effect of a Hungarian made and applied insecticide, organophosphate (Dichlorvos) on antioxidant enzymes and other oxidative and redox parameters of two different fish species, carp (Cyprinus carpio L.) and catfish (Ictalurus nebulosus) was studied. The two fish species have different ways of life. From antioxidant enzymes, changes in superoxide dismutase, catalase, glutathione peroxidase and, in the case of carp, acetylcholinesterase activities were studied in tissue homogenates. Other parameters studied: changes of lipid peroxidation, reduced glutathion and two radicals, which are superoxide and hydroxyl radicals. Our results showed that the organophosphate tested, besides its inhibitory effect on acetylcholinesterase-or together with it-induces changes characteristic of "oxidative stress".