The nucleotide sequence of the Fast-Chateau Douglas isolate of the thermostable alcohol dehydrogenase allele is compared with the sequences of the Slow and Fast alleles of Drosophila melanogaster. Conceptual translation of the FChD sequence indicates that the thermostable polypeptide has the diagnostic FAST amino acid replacement at residue 192 and an additional replacement of serine for proline at residue 214. This suggests a Fast origin for the thermostable Adh allele. However, some of the biochemical properties of the FCHD protein resemble those of the SLOW rather than the FAST polypeptides. The serine for proline replacement confers upon the thermostable polypeptide substrate specificities and some kinetic parameters similar to the SLOW protein. The same replacement substitution within the third coding exon also appears to alter the ADH protein concentration to a level similar to the SLOW polypeptide and the probable effect is at the level of mRNA concentration. The low level of nucleotide sequence variation, other than that leading to the amino acid substitution, suggests a recent origin for the thermostable allele. The time since divergence of the FChD sequence from Fast is estimated to be approximately 260,000-470,000 years.