The rhythms of locomotor activity of male and virgin or mated female flies were compared in the Drosophila melanogaster wild-type strains CantonS, Berlin, and OregonR. Under light-dark conditions, most flies showed a bimodal activity pattern with a morning peak around lights-on and an evening peak before lights-off. For all strains, a distinct sexual dimorphism was observed in the phase of the morning peak. Males had a significantly earlier morning peak than females and consequently a larger phase angle between morning and evening peak (psi(m, e)). Under constant dark conditions, the morning component merged with the evening component to a unimodal activity band in about half of the flies. In those flies who maintained bimodality, the sex-specific difference in psi(m, e) disappeared. Other sex-specific differences were now apparent: Males showed a shorter free-running period than females, and in two of the three strains, females were more active than males. Morning and evening components seem to contribute to the free-running period. Spontaneous or externally provoked change in psi(m, e) were correlated with period changes. In some flies, the morning and the evening components showed splitting, indicating that they are the output of two different oscillators. The sexual dimorphism in the phase of the morning peak under LD-conditions suggests that the function of activity during morning and evening peak might be different, for example, during the morning peak, males are active to find females. Overall, the results underline the multioscillatory nature of Drosophila's circadian system.