ABSTRACT.Mating inDacus tryoniis restricted to dusk, whereas that of a sibling species,Dacus neohumeralis, occurs in the middle of the day. The timing of sexual behaviour in both species is determined by an interaction between a circadian clock and light intensity. InD. tryonipeak mating responsiveness is at the time of dusk, and the optimal light intensity for mating is approximately 91x. InD.neohumeralispeak responsiveness is in the middle of the day, and the optimal light intensity for mating is greater than 10 000 lx. The two species were crossed and the time of mating and response to light intensity of F1, F2and backcross progeny determined. The circadian clock set a mating phase (‘gate’) as narrow in F1flies as in their parents, suggesting the circadian timing mechanism to be common between the two species. The results indicate that the genetic mechanism controlling timing is independent of that controlling response to light intensity, and that both genetic mechanisms are comp