In laboratory studies on the effect of gamma radiation on Anopheles quadrimaculatus Say, it was found that dosages of 8,865 to 12,900 r applied in the pupal or adult stage were required to cause complete sterility. Irradiated females mated to unirradiated males produced no eggs, whereas unirradiated females mated to irradiated males produced a normal number of eggs but none hatched. When irradiated males were introduced into caged populations of normal males and females at ratios of 4: 1:1 or less, usually no reduction in the total number of viable eggs was produced, but at ratios of 6:1:1 and 10:1:1 there was a reduction of about 80%. The dosages(r) required to cause 50% and 100% mortality in 48 hours were; for eggs, 2,600 and >;11,000; for larvae, 32,000 and 120,000; for pupae, 22,000 and 40,000.