To evaluate the sexual competitiveness of irradiated male melon fly, Dacus cucurbitae, the effect of alternate mating on mating receptivity and fertility of the melon fly female was examined. Flies were irradiated two days before adult eclosion with 7 kR of gamma radiation. The receptivity to a second mating of females mated first with irradiated males was almost equal to that of females first mated with normal males. The change in fertility of females mated first with irradiated males and then with normal males was larger than that of females mated first with normal males and then with irradiated males. Fertility of above two types of the females was lower than that of females mated once with normal males. The frequency distribution of percent hatch of eggs laid by females mated successively with normal and irradiated males and vice versa are also described. As there is the tendency to monogamy in the melon fly, irradiated males may work satisfactorily in an eradication program, though the competitiveness of sperm from the irradiated males is thought to be less than that from the normal males.