Effects of gamma radiation from cobalt-60 on prepupae, pupae, and adults of Sitotroga cerealella (Olivier) were determined. Pupal sensitivity depended on age at treatment; the older the pupae, the less susceptible. Irradiation of perpupae and 1-day-old pupae resulted in reduced numbers, delayed and incomplete adult emergence, and in structural deformities including twisted wings and fusion of tarsal and antennal segments. No dose tested on late pupae significantly affected adult emergence. Larvae hatched from the eggs of irradiated 1-day-old adults, but the number of F1 moths was less than in the controls. Females, irradiated at 10 kr as prepupae and mated with normal virgin males or vice versa, either did not oviposit or laid infertile eggs. Irradiation of early pupae at 20 kr may cause complete sterility of both sexes. Following irradiation of late pupae, no dose from 5 to 25 kr prevented hatching of eggs in crosses where treated males were mated to untreated females or vice versa. When males and females, irradiated as 1-day-old moths, were mated, egg deposition and hatchability were markedly reduced. The males remained fertile at all doses.