A novel anaerobic, thermophilic, syntrophic acetate-oxidizing bacterium, strain PB(T), was isolated from a thermophilic (55 degrees C) anaerobic methanogenic reactor which had been treating kraft-pulp waste water. The bacterium oxidized acetate in co-culture with a thermophilic hydrogenotrophic methanogen. Strain PB(T), a gram-positive, spore-forming, rod-shaped bacterium grew optimally at 58 degrees C and pH 6.8. The bacterium grew acetogenically on several alcohols, methoxylated aromatics, pyruvate, glycine, cysteine, formate and hydrogen/CO2. Strain PB(T) also oxidized acetate with reduction of sulfate or thiosulfate as the electron acceptor. The bacterium contained MK-7 as the major quinone. The G+C content of the DNA was 53.5 mol%. Comparative 16S rDNA analysis indicated that strain PB(T) belongs to the Bacillus-Clostridium subphylum. However, it was distant from any known genera or micro-organism. The closest known relative was Thermoterrabacterium ferrireducens with 87.4% similarity. The name Thermacetogenium phaeum gen. nov., sp. nov. is proposed. The type strain is strain PBT (= DSM 12270T).