A new obligately anaerobic, extremely thermophilic, cellulolytic bacterium is described. The strain designated Tp8T 6331 is differentiated from thermophilic cellulolytic clostridia on the basis of physiological characteristics and phylogenetic position within the Bacillus/Clostridium subphylum of the Gram-positive bacteria. Strain Tp8T 6331 is assigned to a new genus Caldicellulosiruptor, as Caldicellulosiruptor saccharolyticus gen., nov., sp. nov.