A strictly anaerobic, thermophilic, gram-positive, spore-forming cubacterium designated strain SERB 5268T (T = type strain) was isolated from an oil field at a depth of 2,100 m, where the temperature was 92 degrees C. The cells of this organism were gram-positive, straight, motile rods (0.5 by 2 to 3 microns) with peritrichous flagella. The cells occurred singly or in pairs during the logarithmic growth phase, but were pleomporphic and filamentous (length, 15 microns) in old cultures. Growth occurred at temperatures of 40 to 75 degrees C, and optimum growth occurred at temperatures between 55 and 60 degrees C. The fermentable substrates included glucose, fructose, galactose, mannose, cellobiose, maltose, sucrose, lactose, D-xylose, D-ribose, mannitol, pyruvate, and starch. The products of fermentation of glucose were lactate, acetate, ethanol, H2, and CO2. The DNA base composition was 35 mol% G+C. The results of 16S rRNA sequence comparisons indicated that strain SEBR 5268T was closely related to Thermoanaerobacter brockii and Thermoanaerobacter finnii, and these three organisms exhibited levels of ribosomal DNA sequence homology of 98 to 99%. The results of DNA-DNA hybridization studies performed with the three organisms confirmed this close affiliation, and as base pairing values of > 70% were obtained, these organisms belong to the same species. Therefore, we propose that T. finnii should be reclassified as a subspecies of T. brockii, Thermoanaerobacter brockii subsp. finnii comb. nov. This automatically creates Thermoanaerobacter brockii subsp. brockii. We also propose that strain SEBR 5268T should be classified as a member of a new subspecies of T. brockii, Thermoanaerobacter brockii subsp. lactiehylicus. The latter differs from T. brockii subsp. brockii and T. brockii subsp. finnii by its 16S rRNA sequence, DNA sequence diversity, lower temperature optimum, G+C content, and carbohydrate utilization spectrum. Strain SEBR 5268T has been deposited in the Deutsche Sammlung von Mikroorganismen as strain DSM 9801T.