The sn-glycerol-3-phosphate dehydrogenase (GPDH, EC 1, 1, 1, 8) locus of Drosophila melanogaster is polymorphic with respect to the number of tandemly duplicated genes in natural populations. The duplicated genes were cloned and the nucleotide sequences were determined. The duplication deletes both the first and second exons and has a size of 4500 b.p. The fact that there is no sequence variation at the junction point of the duplicated units among strains suggests a single origin for the duplication event. Comparison of the nucleotide sequences among the duplicates indicates that the frequent transfer of genetic information occurs from one to the other of the duplicates on the same chromosome either by gene conversion or by unequal crossing over. Because the GPDH duplication is partial and therefore a kind of pseudogene, the observed polymorphism of the number of tandemly duplicated GPDH genes appears to have been driven mainly by random genetic drift.