When two types of Drosophila are in competition, the frequency dependence of mating success routinely is measured in our laboratory by direct observation of mating pairs in "Elens-Wattiaux" observation chambers. The present experiments concern white mutants (used here as a reference standard) in competition with three other genotypes: wild-type Canton S, "giant" gt wa, and "giant" gt13z/w+ Y/C(1)Dx, y f. From the present observations, the frequency dependence of mating success seems a very common phenomenon: a rare-type sexual advantage exists for females and for males. Sexual activity of male and female white mutants is significantly higher when food is present in the mating chamber. Males of the other strains also are more active in the presence of food. Homogamic matings are more frequent among gt13z/w+ Y/C(1)Dx, y f flies.