Expts. were made at Kerrville, Tex., to determine the effects on adult screw-worm flies, Callitroga hominivorax, of treating pupae from the laboratory colony with gamma-rays and X-rays. The test insects were irradiated with gamma-rays from cobalt-60 or with 200 kV X-rays. The irradiation effects were similar. When 6-day-old pupae were treated, males were sterilized by a dose of 2500 r and females by 5000 r. The 5000-r dose appeared not to affect the mating activity of males in cages, but the males did not live as long as normal insects. Sterilized females did not live as long as normal females, but the difference in longevity was not statistically significant. Sterilized females were incapable of producing normal egg masses. Most of those females laid no eggs and the few that oviposited laid only a few scattered eggs instead of the usual masses of approx. 200 eggs. When pupae 3, 4, 5, 6, and 7 days old were irradiated with gamma rays the youngest pupae were most adversely affected so far as pupal mortality and adult longevity were concerned. The oldest pupae were not easily sterilized. Considering the overall effects, the expts. indicated that no dose of less than 5000 r could be depended upon to produce total sterility of both males and females and that it was best to sterilize pupae after they had completed 5 days of development.