ABSTRACT.1The sexual competitiveness of sterile melon fly males [Ducus (Zeugodacus) cucurbitueCoquillett] was estimated under field conditions.2The value of competitiveness was about 0.8 for flies in the fifth to tenth generation from the beginning of mass‐rearing, but decreased to about 0.2 in the eighteenth generation.3The sexual competitiveness estimated using laboratory data was near unity even in the final stage of our study.4It was concluded that the field evaluation of sexual competitiveness is essential to determine the quality of the insects reared for purposes of pest contro