Male melon flies,Dacus cucurbitaeCoquillett, treated with a single dose of the chemosterilant tepa (tris(l‐aziridinyl) phosphine oxide), or with gamma irradiation, either single or fractionated doses, did not differ significantly in sexual competitiveness as determined by percentage hatch of eggs. Mating competitiveness of males treated by either method ranged from 53 to 66% of that of untreated males. In another study, melon flies (males and females) sterilized with 0.0125% tepa, the threshold dose for both sexes, completely suppressed a population when the ratio was 16:16:1:1 (sterile males‐sterile females‐untreated males‐untreated females) as determined by no egg hatch.