HOUSEFLIES metabolize dialkyl phosphorothionate insecticides, such as diazinon and parathion, to the corresponding dialkyl phosphorothioic acids1,2. This reaction is thought to be controlled by gene a, on chromosome II (ref. 3), associated with low aliesterase activity4. The cleavage mechanism is reported to be in the microsomes2,5,6. I have found three detoxification mechanisms in subcellular fractions of six strains (Table 1) of housefly and related these to the genetics of diazinon-resistance7,8 of the flies.