Stable strains of Drosophila melanogaster flies producing mosaic sex-pheromones were created by manipulating the sex-determination gene transformer in different cellular patterns. Chromosomally male flies produced molecules that were partly or totally feminized, depending upon the strain. The influence on male courtship behaviour of live flies producing different mixtures of male and female pheromones was measured. The intensity of the male response was mainly a function of the stimulatory effect of 7, 11 nonacosadiene and 7, 11 heptacosadiene, two female pheromones, combined with the inhibiting effect of 7-tricosene, the male predominant molecule. These compounds were effective at very different doses. The influence of secondary molecules on male behaviours was also evaluated. The role of these substances is discussed with regard to their structure, sexual specificity, and evolutionary function.