Six naturally occurring but rare alleles of sn-glycerol-3-phosphate dehydrogenase (Gpdh) in Drosophila melanogaster have been investigated in this study. They all belong to a class of GpdhUF (ultra-fast) alleles, because their electrophoretic mobilities are faster than that of the GpdhF (fast) allele. The GpdhUF variants are widespread, and have been reported from five continents. DNA sequence analysis has shown that the change in electrophoretic mobility was in each allele caused by a single amino acid residue substitution in the encoded protein. In the XiamenUF allele it is a substitution of lysine (AAA) to asparagine (AAT) in exon 1 (residue 3). An asparagine (AAT) to aspartate (GAT) change was found in exon 6 (residue 336) in the IowaUF and NetherlandsUF alleles. The mobility of the RaleighUF allele was altered by a valine (GTG) to glutamate (GAG) substitution in exon 3 (residue 76). Two mutations were detected in the BrazzavilleUF allele: a lysine (AAG) to methionine (ATG) substitution in exon 2 (residue 68) is responsible for the ultra-fast phenotype of this variant, while a tyrosine (TAT) to phenylalanine (TTT) substitution in exon 4 (residue 244) is not expected to alter the electrophoretic mobility of the encoded protein. These results indicate that the GpdhUF alleles originate from different mutational events, and only two of them--IowaUF and NetherlandsUF--might share a common ancestry. The GPDH activity of the IowaUF allele is intermediate between those of the GpdhS and GpdhF control stocks. The other GpdhUF variants have lower activities than the controls: XiamenUF--83%, RaleighUF--80% and BrazzavilleUF--73% of the GpdhF control.