Experiments were conducted to determine the effects of piperonyl butoxide, a synthetic methylenedioxphenyl inhibitor of cytochrome(s) P450, on the toxicity of organophosphate insecticides to three cladoceran test species: Ceriodaphnia dubia, Daphnia magna, and Daphnia pulex. Coadministration of piperonyl butoxide effectively reduced the acute toxicity of four metabolically activated organophosphates (parathion, methyl parathion, diazinon, and malathion) and did not affect the toxicity of three organophosphates not requiring metabolic activation (dichlorvos, chlorfenvinphos, and mevinphos). These results indicate that piperonyl butoxide may be an effective tool in toxicological research focused upon identifying specific compounds responsible for toxicity in complex aqueous mixtures. (Copyright (c) 1991 Academic Press, Inc.)