Irradiating 6-d-old Musca autumnalis De Geer pupae in nitrogen with 500-2,500 rads did not affect adult emergence, female fecundity, or male competitiveness. Male and female fertility decreased with increased dose, reaching about 1% at 2,500 rads. Egg to adult survival upon backcrossing irradiated males or females to unirradiated flies was only 7% at 1,500 rads and 0.7% at 2,000 rads. Doses of up to 2,000 rads did not affect male survival, but males treated with 2,500 rads had reduced survival rates. These results are better than those with irradiation in air and indicate that irradiation with 1,800-2,000 rads in nitrogen would maximize dominant lethality and minimize fitness loss.