The metabolic fate of parathion and diazinon, in one susceptible and two orgarophosphate-resistant strains of houseflies, was studied by chromatography and radioisotope techniques. The main interstrain difference resulted from the superior ability of the resistant strains to degrade parathion to diethyl phosphorothionaie. The enzyme involved in the degradation process in the strains S and Ka was purified about 30 times by acetone powder formation, DEAE-fractionation, and ethanol precipiiation. The partially purified enzyme preparations obtained from all three strains hydrolyzed parathion and diazinon to diethyl phosphorothionaie, but their activity in hydrolyzing paraoxon was relatively low. The importance of these interstrain differences in relation to organophosphate resistance is discussed.TUDIES on insecticide resistance dur-S ing the past decade have elucidated several mechanisms of resistance in many insect species ( 3 ) . Biochemical aspects of organophosphate-resistance (OP-resistance) were first studied in the housefly (Musca dome.rtica) by March ( 7 4 , who found the malathion-resistant Stauffer strain to degrade malaoxon in vitro more rapidly than the susceptible individuals. Similarly. Oppenoorth and van Asperen (27) showed the importance of biochemical degradation of paraoxon, diazinon, and malaoxon by demonstrating that, a t relatively low concentrations, the speed of disappearance of these toxic phosphates was much faster in homogenates of resistant strains than that in the susceptible strain. They employed a fly head cholinesterase bioassay technique. These authors had already shown by genetic and other means that OP-resistance was very often associated with an abnormally low level of aliesterase activity in the housefly (7). They concluded that the mutant gene which caused the lo\.z.er aliesterase level Present address: Department of Entomology, Cornel1 Cniversitv, Ithaca,