Extremely thermophilic bacteria were isolated from sediments collected at the Guaymas Basin hydrothermal vent located in the Gulf of California. One isolate (FC89) is a hydrogenotrophic methanogen with an optimal growth temperature of 85-degrees-C; this isolate appears to be closely related to the previously described Methanococcus jannaschii. Thermophilic isolates TY and TYS are heterotrophic, sulfur-reducing archaea that differ from other thermophilic heterotrophic strains in physiological and molecular properties. Both heterotrophic isolates fermented carbohydrates and proteinaceous substrates; acetate was the primary product of carbohydrate fermentation, whereas acetate and a mix of organic acids were primary products of proteinaceous substrate fermentation. A detailed microbiological characterization of the isolates and a profile of fermentable substrates and fermentation products are described.