SUMMARYFrom the F2ratios of crosses betweenDrosophila melanogasterstrains homozygous for theAdhF(F) and theAdhs(S) allele it has been concluded that the developmental time ofFFhomozygotes is shorter than that ofSShomozygotes. This difference is found to be reinforced by increasing levels of crowding. A further analysis of developmental times has been performed by the transfer of larvae to agar medium after they have stayed for periods of variable length on regular food. From the percentage of emerging adults it can be concluded thatFFlarvae and, to a lesser extentFSlarvae, either reach their critical weights for pupation earlier thanSSlarvae or possess a lower critical weight. These differences in developmental time influence the course of allele frequencies. Between populations kept on a 2-week transfer schedule and on a 3-week schedule a divergence of allele frequencies is observed, in the former a decrease inSfrequency occurs. The relevance of the observed differences in developmental time for the maintenance of theAdhpolymorphism is discussed for laboratory populations kept on regular food and at varying densities.