Housefly strains resistant to insecticides by oxidative detoxication mechanisms possess higher levels of microsomal cyclodiene epoxidase activity as compared with two susceptible or one Dieldrin-resistant strain. The epoxidase system in strains with higher activity has apparently greater affinity for aldrin, isodrin and heptachlor, as judged by the Michaelis constants. The epoxidase activity in resistant strains increases after the emergence of adult flies and maximum activity occurs at slightly different ages in each strain. Male flies have about half of the activity of the females. The high epoxidase activity in an Isolan-resistant strain is inherited through a semidominant gene, Ox, on chromosome II.