Abstract
Summary: Sexual isolation between the sibling speciesD. simulansandD. mauritianais due largely to the rejection ofD. simulansmales byD. mauritianafemales. Genetic analysis shows that genes on the X and third chromosomes contribute to the differences between males causing sexual isolation, while the Y chromosome, second chromosome and cytoplasm have no effect. These chromosome effects differ from those observed in a previous analysis of sexual isolation in hybrid females, implying that different genes cause sexual isolation in the two sexes.