Studies with 9 insecticide chemicals establish that gene (s) on autosome 2 in one resistant house fly, Musca domestica L., strain (R-Baygon) and on autosome 5 in another (R-Fc) control the level of activity of reduced nicotinamide-adenine dinucleotide phosphate (NADPH)-dependent oxidases which detoxify certain chlorinated hydrocarbon, pyrethroid, organophosphate, and methylcarbamate insecticides. Resistance to several of these insecticides in each strain is conferred, in part, by gene (s) on the same autosome as that conferring high oxidase activity. Apparently, a small number of mutants in the house fly control the level of oxidative metabolism of many insecticide chemicals and a portion of the resistance to them.