We investigated the effect of adult density on male and female longevity and behavioural interactions in a mass-reared strain of the medfly. Male survival decreased significantly, and male–male behavioural interactions increased significantly, with increasing male density (males were kept at 1, 2, 6 and 10 flies per 285 cm2in pots of 5.5 cm in diameter and 12 cm high). No such effects were observed in females over the same density range. This suggests that male survival decreased in response to the deleterious effects of male–male behavioural interactions. At the highest density, courtship wing fanning directed towards other males decreased significantly over time. In addition, as either density or male-biased sex ratio increased, males were increasingly likely to be engaged in courtship wing fanning that was not directed towards females. These results suggest that high adult density may significantly reduce male reproductive success, by decreasing survival and mating success. The results highlight the potentially high survival costs of male–male interactions and indicate differences in the nature of reproductive costs in the two sexes.