The hyperthermophilic Archaea represent some of the most ancient organisms on earth. A study of enzymatic cofactors in these organisms could provide basic information on the origins of related cofactors in man and other more recently evolved organisms. To this end, the nature of the tungsten cofactor in aldehyde ferredoxin oxidoreductases from Pyrococcus furiosus and ES-4 and in formaldehyde ferredoxin oxidoreductases from P. furiosus and Thermococcus litoralis has been investigated. All four proteins contain molybdopterin, previously characterized as the organic component of the molybdenum cofactor in a large number of molybdoenzymes. Molybdopterin was identified by conversion to the dicarboxamidomethyl derivative by alkylation of the vicinal sulfhydryl groups on the pterin side chain and by conversion to the oxidized fluorescent derivative, Form A. The pterin of the tungsten cofactor in the four enzymes was examined for the presence of appended GMP, CMP, AMP, or IMP previously observed in molybdenum cofactors of some molybdoenzymes. No evidence for the presence of a molybdopterin dinucleotide or other modified form of molybdopterin was obtained. These results further document the essential nature of molybdopterin for the function of molybdenum and tungsten enzymes in diverse life forms.