Abstract The archaeon Pyrococcus furiosus grows optimally at 100°C during the fermentation of peptides and carbohydrates to yield organic acids, CO2 and H2. It also reduces elemental sulfur (S°) to H2S. The production of H2 is catalyzed by a Ni-containing hydrogenase which also functions as a sulfur reductase [Ma et al. (1993) Proc. Natl. Acad. Sci. 90, 5341–5344]. It is shown here that this bifunctional enzyme, termed sulhydrogenase, uses NADPH rather than ferredoxin (or NADH) as an electron donor. The disposal of excess reductant during fermentation is proposed to occur via ferredoxin-linked oxidoreductases, a NADP: ferredoxin oxidoreductase and sulfhydrogenase.