SummaryIncipient sexual isolation between two strains ofD. metanogasterwas tested by male choice experiments. During ten years (150 generations) one strain, named Se, was reared in a food medium containing 10‐‐4M sodium selenite, while the other, named Co, was kept under normal conditions. Both strains were derived from the same natural population. There was no evidence of preference for a mate of the same strain. There was an interaction between type of female and duration of time available for mating. Se males inseminated fewer females and fewer Se females were inseminated over