When male pupae ofTrichoplusia ni(Hübner) were irradiated with 15 krad of gamma irradiation, the dose that effectively induces sterility in the progeny, the performance of emerging adults was inferior to that of males irradiated similarly as adults. However, when the same dose to pupae was fractionated, that is, applied over several successive days, the performance of emerging adults was comparable to that of males exposed as adults to an unfractionated dose, and the performance of the F1progenies did not differ. Thus, the male parents were about 50% sterile, the F1males were about 90% sterile, and the F1females were about 80% sterile. Adult females from pupae irradiated with the fractionated dose were 100% sterile.