ABSTRACT Twelve laboratory populations of recently collected Drosophila willistoni were begun with different frequencies of alleles at three enzyme loci, six populations at 25° and six at 19°. Periodic sampling of the populations allowed monitoring of the frequency changes in allozymes over time.—At Lap-5 (a locus coding for leucine amino peptidase), three alleles converged to the same frequencies in all populations at both temperatures. The apparent equilibrium frequency of the major allele was about.75; this is different from the frequency (.57) found in the natural population from which the experimental populations were begun. Allele frequency changes at the esterase-5 locus (Est-5) were slower but consistent in all cages. It is difficult to determine if an equilibrium has been reached. However, the frequency of the rare allele in all cages is about the same as in wild populations, 5%. Alleles at both Lap-5 and Est-5 are non-randomly associated with inversions in the chromosomes onto which they map. Because of these associations, it is impossible to unambiguously attribute the change in allele frequencies to selection at the loci being observed.—After one year, no significant gene frequency changes were detected at Est-7, the third locus studied.