Competitive inhibition of aldrin epoxidation by O-dealkylation of p-nitroanisol and methoxychlor; hydroxylation of photoisodrin, photodieldrin, aniline, and hexobarbital; and hydrolysis of parathion indicates that all these reactions take place at the same site on Musca domestica L. microsomal oxidase. These reactions are inhibited by very low concentrations of sesamex. The mixed function oxidase apparently also performs O-dehydrochlorination of photoaldrin, photodieldrin, and photoheptachlor, and hydrolysis of the WARF-antiresistant (N,N-dibutyl-p-chlorobenzenesulfonamide). The gene, Ox, controlling the activity of the mixed function oxidase is located on chromosome 2 in the dimetilan-selected and the Orlando DDT-resistant strains of the house fly.