Abstract According to ecological and behavioural studies, Drosophila simulans is considered to be less tolerant of darkness than its sibling species D. melanogaster which is well adapted both behaviourally and physiologically to darkness. The relationships between physiological and behavioural adaptations have been analysed by studying the developmental and the reproductive capacities of D. simulans submitted to various light regimes (LL, LD 12:12, DD). This species has a lower reproductive capacity than D. melanogaster but failed to react to light treatment. In particular, D. simulans showed no effect of darkness on either fertilization or ovarian function. The lack of differences between D. melanogaster and D. simulans as regards their physiological capacities in relation to light regime suggests that the selective pressures of light may act at different levels of regulation.