Comparisons of "time of mating" (the time copulation begins) between lines selected for short and long developmental periods have been made in the melon fly, Bactrocera cucurbitae. These comparisons showed that longer development periods were associated with later initiation of mating. Crosses were also made between selected lines to ascertain the genetic basis of developmental period and time of mating. Comparisons of daily activity rhythms for four types of behavior (locomotion, preening, feeding, and wing vibration) between the selected lines showed the following; (1) locomotion and preening occurred in daytime for the short developmental period lines, versus mainly at night for the long developmental period lines; (2) feeding behavior occurred in daytime for both the short and the long developmental period lines; and (3) wing vibration, a component of courtship behavior of males, occurred at dusk for the short developmental period lines and at night for the long developmental period lines.