An anaerobic thermophilic bacterium, designated strain JW/SA-NV4(T), was isolated from a xylan-supplemented enrichment culture from Trego hot spring located within the Black Rock Desert (NV, USA). Cells were generally straight or slightly bent rod-shaped, 0.4-0.8 microm in width and 3-6 microm in length during exponential growth. Cells from stationary phase were variable in size and shape, showing curved or bent morphology. Motility was not seen and flagella were not observed in electron micrographs. Sporulation was not observed. Strain JW/SA-NV4(T) stained Gram-negative but is phylogenetically Gram-type positive. Growth occurred at pH(25 degrees C) 6.8-8.8, with optimum growth at pH 8.4; no growth occurred at pH 9.0 or above or at 6.5 or below. With glucose or xylose as the carbon source, strain JW/SA-NV4(T) grew at 44-74 degrees C; no growth occurred at 76 degrees C or above or at 42 degrees C or below. However, the optimum temperature was 62 and 66 degrees C when grown on glucose and xylose, respectively. The shortest doubling time observed with glucose was approximately 4 h, and with xylose approximately 3.4 h. Strain JW/SA-NV4(T) tolerated an atmosphere containing up to 0.1 % O(2); no growth occurred at a gas atmosphere of 0.2 % O(2). Chemo-organotrophic growth occurred with xylose, glucose, mannose, xylan, pyruvate, fructose, ribose, Casamino acids, mannitol, tryptone, peptone, cellobiose and yeast extract. When grown in mineral media containing 1 g yeast extract l(-1) as an electron donor, thiosulfate and sulfur were reduced to sulfide. The G+C content of the DNA was 38.6 mol% (HPLC). 16S rRNA gene sequence analysis placed strain JW/SA-NV4(T) within the order Thermoanaerobacterales and within the Thermoanaerobacterales Incertae Sedis Family III, specifically between taxa classified within the genera Thermosediminibacter and Thermovenabulum. The closest phylogenetic neighbours were Thermosediminibacter oceani JW/IW-1228P(T) (94.2 % 16S rRNA gene sequence similarity) and Thermosediminibacter litoriperuensis JW/YJL-1230-7/2(T) (94.0 %) [Lee, Y.-J., Wagner, I. D., Brice, M. E., Kevbrin, V. V., Mills, G. L., Romanek, C. S. & Wiegel, J. (2005). Extremophiles 9, 375-383]. Based on physiological and genotypic characteristics, strain JW/SA-NV4(T) (=DSM 18802(T)=ATCC BAA-1454(T)) is proposed to represent the type strain of a novel species in a novel genus, Caldanaerovirga acetigignens gen. nov., sp. nov.