Drosophila melanogaster gynandromorphs were introduced into a mating chamber containing two wild type males. From the differential courtship responses of these males it could be concluded that the only important factor which enables a male to distinguish between conspecific males and females and to direct persistent courtship only toward females is tissue composition of females. This fact is interpreted in terms of a female-specific sex pheromone. Most of this pheromone is produced by cells whose precursors on the blastoderm surface are very close to, or identical with, the blastoderm precursors of the tergites. Some of this pheromone is probably produced by cells whose precursors are very close to, or identical with, the precursors of the sternites and thorax.