The crystal structure of the tungsten-containing aldehyde ferredoxin oxidoreductase (AOR) from Pyrococcus furiosus , a hyperthermophilic archaeon (formerly archaebacterium) that grows optimally at 100°C, has been determined at 2.3 angstrom resolution by means of multiple isomorphous replacement and multiple crystal form averaging. AOR consists of two identical subunits, each containing an Fe 4 S 4 cluster and a molybdopterin-based tungsten cofactor that is analogous to the molybdenum cofactor found in a large class of oxotransferases. Whereas the general features of the tungsten coordination in this cofactor were consistent with a previously proposed structure, each AOR subunit unexpectedly contained two molybdopterin molecules that coordinate a tungsten by a total of four sulfur ligands, and the pterin system was modified by an intramolecular cyclization that generated a three-ringed structure. In comparison to other proteins, the hyperthermophilic enzyme AOR has a relatively small solvent-exposed surface area, and a relatively large number of both ion pairs and buried atoms. These properties may contribute to the extreme thermostability of this enzyme.