Recent sequencing of over 2300 nucleotides containing the alcohol dehydrogenase (Adh) locus in each of 11 Drosophila melanogaster lines makes it possible to estimate the approximate age of the electrophoretic "fast-slow" polymorphism. Our estimates, based on various possible patterns of evolution, range from 610,000 to 3,500,000 years, with 1,000,000 years as a reasonable point estimate. Furthermore, comparison of these sequences with those of the homologous region of D. simulans and D. mauritiana allows us to infer the pattern of evolutionary change of the D. melanogaster sequences. The integrity of the Adh-f electrophoretic alleles as a single lineage is supported by both unweighted pair-group method (UPGMA) and parsimony analyses. However, considerable divergence among the Adh-s lines seems to have preceded the origin of the Adh-f allele. Comparisons of the sequences of D. melanogaster genes with those of D. simulans and D. mauritiana genes suggest that the split between the latter two species occurred more recently than the divergence of some of the present-day Adh-s genes in D. melanogaster. The phylogenetic analyses of the D. melanogaster sequences show that the fast-slow distinction is not perfect, and suggest that intragenic recombination or gene conversion occurred in the evolution of this locus. We extended conventional phylogenetic analyses by using a statistical technique for detecting and characterizing recombination events. We show that the pattern of differentiation of DNA sequences in D. melanogaster is roughly compatible with the neutral theory of molecular evolution.