Genetic differences that influence mating preferences were studied in genetically defined lines of Drosophila melanogaster. Initial results suggested substantial differences between two types of females with respect to the types of male preferred as mates, but further experimentation showed that the mating patterns were conditional on the mode of anesthesia (CO2 versus ether). In a statistical test of independence, the major determinant of mating choice in these experiments was due to an interaction effect between genotypes and mode of anesthesia. The observations might be explained by the differential sensitivity of male genotypes to ether. Etherization at emergence has lasting effects on mating behavior; it alters not only the latency and frequency of mating 4 days later, but also the pattern of matings observed.