Oxidative phosphorylation involves the coupling of ATP synthesis to the proton-motive force that is generated typically by a series of membrane-bound electron transfer complexes, which ultimately reduce an exogenous terminal electron acceptor. This is not the case with Pyrococcus furiosus, an archaeon that grows optimally near 100 degrees C. It has an anaerobic respiratory system that consists of a single enzyme, a membrane-bound hydrogenase. Moreover, it does not require an added electron acceptor as the enzyme reduces protons, the simplest of acceptors, to hydrogen gas by using electrons from the cytoplasmic redox protein ferredoxin. It is demonstrated that the production of hydrogen gas by membrane vesicles of P. furiosus is directly coupled to the synthesis of ATP by means of a proton-motive force that has both electrochemical and pH components. Such a respiratory system enables rationalization in this organism of an unusual glycolytic pathway that was previously thought not to conserve energy. It is now clear that the use of ferredoxin in place of the expected NAD as the electron acceptor for glyceraldehyde 3-phosphate oxidation enables energy to be conserved by hydrogen production. In addition, this simple respiratory mechanism readily explains why the growth yields of P. furiosus are much higher than could be accounted for if ATP synthesis occurred only by substrate-level phosphorylation. The ability of microorganisms such as P. furiosus to couple hydrogen production to energy conservation has important ramifications not only in the evolution of respiratory systems but also in the origin of life itself.