A thermophilic anaerobic bacterium designated strain RWcit2T (T = type strain) was isolated from the production water of a petroleum reservoir. The cells of this organism are straight to slightly curved rods that are gram negative and nonmotile. Spore formation has not been demonstrated. Growth occurs at temperatures ranging from 28 to 60 degrees C, with optimum growth occurring at 55 degrees C, and at pH values ranging from 5.5 to 8.6, with optimum growth occurring between pH 7 and 7.6. Growth occurs in media containing 0 to 20 g of NaCl per liter, and optimum growth occurs in the presence of 10 g of NaCl per liter. Strain RWcit2T grows on a range of organic acids, including citrate, pyruvate, malate, fumarate, and tartrate; on protein extracts; and on a limited number of carbohydrates. Sulfur, thiosulfate, and cystine are reduced to hydrogen sulfide. Sulfate, sulfite, and nitrate are not reduced. The DNA base composition is 44 mol% G + C. The 16S ribosomal DNA sequence revealed that strain RWcit2T is a member of the domain Bacteria and forms a branch that is approximately equidistant from Dictyoglomus thermophilium and Thermoanaerobacter spp. (level of similarity, 82%). Strain RWcit2T cannot be placed in any previously described taxon based on its phylogenetic and physiological traits and is named Anaerobaculum thermoterrenum gen. nov., sp. nov.