Twenty-five samples of stratal fluids obtained from a high-temperature (60-84 degrees C) deep subsurface (1700-2500 m) petroleum reservoir of Western Siberia were investigated for the presence of dissimilatory Fe(III)-reducing microorganisms. Of the samples, 44% and 76% were positive for Fe(III) reduction with peptone and Hz respectively as electron donors. In most of these samples, the numbers of culturable thermophilic Hz-utilizing iron reducers were in the order of 10-100 cells/ml. Nine strains of thermophilic anaerobic bacteria and archaea isolated from petroleum reservoirs were tested for their ability to reduce Fe(III). Eight strains belonging to the genera Thermoanaerobacter, Thermotoga, and Thermococcus were found capable of dissimilatory Fe(m) reduction, with peptone or H-2 as electron donor and amorphous Fe(III) oxide as electron acceptor. These results demonstrated that Fe(m) reduction may be a common feature shared by a wide range of anaerobic thermophiles and hyperthermophiles in deep subsurface petroleum reservoirs.