The insecticides aldrin and heptachlor have been found to epoxidize to dieldrin and heptachlor epoxide, respectively, in soils treated for control of soil-inhabiting insects. Such conversions may he assumed to be advantageous since the compounds produced are not only toxic but also are more residual than are the parent materials. This may partially explain the prolonged insecticidal efficiency obtained in soils treated with aldrin and heptachlor.