Courtship song and mating speed of hybrids between Drosophila melanogaster and D. simulans were investigated. The courtship song of hybrid males is identical to that of D. simulans, suggesting that X chromosome determination, known from the cross between D. pseudoobscura and D. persimilis, is also possible here. Wingbeat frequency of hybrids is intermediate between that of the two parents, demonstrating that courtship song and wingbeat frequency are inherited independently of each other. In mating test, hybrid males cout and are accepted by D. simulans females more than hybrid females (presumably because their song is more "acceptable" to the former). D. melanogaster males readily, hybrids less readily, and D. simulans least.