Several strains of heterotrophic, anaerobic thermophilic bacteria were isolated from hot springs of the Uzon Caldera, Kamchatka, Far East Russia. Strain JW/IW010(T) was isolated from a hot spring within the West sector of the Eastern Thermal field, near Pulsating Spring in the Winding Creek area. Cells of strain JW/IW010(T) were straight to slightly curved rods, 0.5 mum in width and variable in length from 2 to 5 mum and occasionally up to 15 mum, and formed oval subterminal spores. Cells stained Gram-negative, but were Gram-type positive. Growth was observed between 32.5 and 69 degrees C with an optimum around 61 degrees C (no growth occurred at or below 30 degrees C, or at or above 72 degrees C). The pH(60 degrees C) range for growth was 4.2-8.9 with an optimum at 7.1 (no growth occurred at or below pH(60 degrees C) 3.9, or at 9.2 or above). The shortest observed doubling-time at pH(60 degrees C) 6.9 and 61 degrees C was 30 min. Strain JW/IW010(T) was chemo-organotrophic; yeast extract, peptone, Casamino acids and tryptone supported growth. Yeast extract was necessary for the utilization of non-proteinaceous substrates, and growth was observed with inulin, cellobiose, maltose, sucrose, glucose, fructose, galactose, mannose, xylose, trehalose, mannitol, pyruvate and crotonate. The G+C content of the genomic DNA of strain JW/IW010(T) was 33.6 mol% (HPLC method). The major phospholipid fatty acids were iso-15 : 0 (53.5 %), 15 : 0 (11.8 %), 16 : 0 (7.3 %), 10-methyl 16 : 0 (7.3 %) and anteiso-15 : 0 (5.3 %). 16S rRNA gene sequence analysis placed strain JW/IW010(T) in the genus Thermoanaerobacter of the family 'Thermoanaerobacteriaceae' (Firmicutes), with Thermoanaerobacter sulfurigignens JW/SL-NZ826(T) (97 % 16S rRNA gene sequence similarity) and Thermoanaerobacter kivui DSM 2030(T) (94.5 %) as the closest phylogenetic relatives with validly published names. The level of DNA-DNA relatedness between strain JW/IW010(T) and Thermoanaerobacter sulfurigignens JW/SL-NZ826(T) was 64 %. Based on the physiological, phylogenetic and genotypic data, strain JW/IW010(T) represents a novel taxon, for which the name Thermoanaerobacter uzonensis sp. nov. is proposed. The type strain is JW/IW010(T) (=ATCC BAA-1464(T)=DSM 18761(T)). The effectively published strain, 1501/60, of 'Clostridium uzonii' [Krivenko, V. V., Vadachloriya, R. M., Chermykh, N. A., Mityushina, L. L. & Krasilnikova, E. N. (1990). Microbiology (English translation of Mikrobiologiia) 59, 741-748] had approximately 88.0 % DNA-DNA relatedness with strain JW/IW010(T) and was included in the novel taxon.