This paper describes a perturbation experiment on the frequency of the F and S Alcohol dehydrogenase (Adh) alleles of D. melanogaster. Fifty-four iso-female lines set up from three wild populations and with initial F frequencies of either 0.25, 0.50 or 0.75 were maintained on standard laboratory food medium at 22 degrees. At generations 4, 12 and 20 the lines were again scored for Adh gene frequencies. Maximum likelihood procedures were used to estimate selection coefficients for the Adh genotypes. An analysis of deviance was used to compare the coefficients against expectations under the hypotheses of neutrality and of constant values for the three base populations, and for the three initial gene frequency classes. Highly-significant departures from neutrality were observed; over all 54 lines, the set of relative fitnesses for S/S:F/S:F/F was estimated as 1.00:1.08:1.08. In addition, there were significant differences between lines in the outcome of selection which were not attributable to differences between base populations or initial F frequencies. These residual between-line differences, as well as some between-generation, within-line differences are discussed in terms of linkage disequilibria with background genes and electrophoretically cryptic variation at the Adh locus.