Genome sequence of Pyrobaculum calidifontis, a hyperthermophilic archaeon, harbors three open-reading frames annotated as alcohol dehydrogenases. One of them, Pcal_1311, does not display a significantly high homology with any of the characterized alcohol dehydrogenases. Highest homology of 38% was found with the characterized counterpart from Geobacillus stearothermophilus. To examine the biochemical properties of Pcal_1311, we have cloned and functionally expressed the gene in Escherichia coli. Purified recombinant Pcal_1311 catalyzed the NAD(H)-dependent oxidation of various alcohols and reduction of aldehydes, with a marked preference for substrates with functional group at the terminal carbon. Highest activity for the oxidation reaction (3 mu mol min(-1) mg(-1)) was found with 1,4-butanediol and for the reduction reaction (150 mu mol min(-1) mg(-1)) with glutaraldehyde. Both the oxidation and reduction activities increased with the increase in temperature up to 80 A degrees C. Recombinant Pcal_1311 was highly stable and retained more than 90% activity even after incubation of 180 min at 90 A degrees C. In addition to the thermostabilty, Pcal_1311 was highly stable in the presence of known denaturants including urea and guanidine hydrochloride. The high stability, particularly thermostability, and the NADH-dependent aldehyde reduction activity make Pcal_1311 a unique member in the alcohol dehydrogenase family.