ABSTRACT We have identified the effects of genes that regulate sex determination on female-specific tissues in the abdomen that produce sex pheromones and parts of the central nervous system that function when a male performs courtship. To do this, we monitored the sexual behaviors of flies with mutations in the transformer (tra), doublesex (dsx) and intersex (ix) genes. Except for tra, which transforms diplo-X flies so that they look and function like normal males, these mutations do not have the same effect on pheromone-producing tissues and the central nervous system as they do on the appearance of the fly. The dsx and ix mutations, which make diplo-X-flies look like intersexes, do not transform the flies so that they can perform courtship, suggesting that these genes do not regulate the development of sex-specific parts of the central nervous system. Conversely, the ix mutation, which has no effect on the appearance of haplo-X flies, makes the flies sexually attractive and impairs their ability to perform courtship, which implies that the ix gene is active in internal tissues of males.