When males ofCeratitis capitata(Wiedemann) were irradiated either in the late pupal (2 days before eelosion) or the adult stage (1 to 24 hours old) with 7, 9, and 11 krad in air and in nitrogen, exposure to nitrogen before and during irradiation decreased sterility. No significant differences in sterility were found among nitrogen exposure periods of 5, 15, 25 or 35 minutes before irradiation. Adult longevity decreased with increasing radiation dose regardless of whether the adults or pupae were irradiated in air or nitrogen. However, at any of the doses, individuals irradiated in nitrogen as pupae were longer lived than comparable pupae irradiated in air. By contrast, those irradiated in nitrogen as adults lived as long as when irradiated in air. Competitiveness of males irradiated as adults in nitrogen atmosphere was higher than that for males irradiated in air, but the differences were not statistically significant. Males irradiated as pupae in nitrogen were significantly more competitive than those irradiated in air. Differences between competitiveness of males irradiated in air as pupae and males irradiated as adults were not significant. In contrast, when irradiation was applied in nitrogen atmosphere, competitiveness of males irradiated as adults was significantly less than that of males irradiated as pupae.