In the course of some experiments with Drosophila melanogaster on the adaptive value of several dominant mutants in competition with their wild-type alleles, an anomalous situation arose with respect to Lobe-2. Further investigation of these results produced an explanation which raises some interesting questions about reports of heterosis in Drosophila. In five different laboratory populations, the second chromosome dominant mutant, L2 was very rapidly eliminated when in competition with its wild-type alleles. Homozygous L2/L2 stocks were easily maintained, but the L2 mutant was clearly at a selective disadvantage in the populations. In order to test the various components of fitness such as viability, mating success, longevity, and so on, flies were removed from the populations. With their progeny,