Abstract In connexion with a detailed study of the effects of gamma irradiation on the gypsy moth, histological examination was made of normal larvae in instars I to V and pupae to determine the timing and sequence of spermatogenesis. Similar examination was made of pupae that had been irradiated as instar IV and V larvae with dosages ranging from 500 to 20,000 roentgens. Spermatogenesis occurred in the larval period, and meiotic activity began in instar IV. Fully formed spermatozoa were first observed in instar V. The appearance of gonadal tissues of treated and untreated specimens in the pupal stage is described. All irradiation dosages caused some cytological damage; instar IV larvae exposed to 20,000 r died in 7–10 days. The major gross effects of irradiation were: degeneration of germinal cysts, shattering of sperm bundles within the follicles, plugging of the entrance to the vas deferens by interstitial and other tissue (this occurred with higher dosages only), thickening of testes walls and septa and of the vas deferens, and clumping of fat body cells around the testes. One or more of these morphological changes might be related to the sterile condition.