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[en] The sterile insect technique (SIT) has been used successfully against the medfly. The use of gamma irradiation for sterilizing insects has some adverse effects on their competitiveness which in turn reduces the efficiency of the technique. This study was carried out to evaluate the radioprotective effects of melatonin (N-acetyl-5-methoxytyrptamine) on egg hatchability, larval duration, pupal recovery, pupal weight, adult recovery, sex ratio, adult survival, flight ability and mating competitiveness of the medfly, Ceratitis capitata (Wied.), sterile males against the oxidative stress induced by gamma radiation. The results showed that the use of melatonin before the application of radiation doses did not affect the percentages of egg hatchability, larval duration and pupal weight while the addition of melatonin to the larval diet led to a significant increase in the percent of the pupal recovery as compared with the control group. The results also showed significant increases in adult recovery, adult survival and flight ability of males reared on larval diet containing melatonin before irradiation as compared to those treated with radiation alone. The competitiveness values were drastically decreased (0.30, 0.17 and 0.19) with the increase of gamma irradiation (90, 100 and 110 Gy, respectively). However, the results showed that the competitiveness values were increased in males previously reared on a larval diet containing 1 g melatonin/kg larval media (0.1%) and the produced full grown pupae were irradiated with 90, 100 and 110 Gy. The highest male mating competitiveness was recorded when melatonin was added to the larval diet then irradiated the produced pupae with the sterilizing dose (90 Gy). The results showed that the competitiveness values were 0.67, 0.47 and 0.46 for the doses 90, 100 and 110 Gy, respectively. The results refer to availability of using melatonin as a simple and effective means to improve the quality and the male mating competitiveness of the medfly adult males for release in SIT programs.