The thermophilic, cellulolytic, anaerobic bacterium 'Anaerocellum thermophilum' strain Z-1320 was isolated from a hot spring almost two decades ago and deposited in the German culture collection (DSMZ) as DSM 6725. The organism was classified as a new genus Anaerocellum primarily based on growth physiology, cell wall type and morphology. The results of recent physiological studies and of phylogenetic and genome sequence analyses of the strain of 'A. thermophilum' obtained from the DSMZ (DSM 6725), henceforth referred to as strain DSM 6725, show that its properties differ from those originally described for strain Z-1320. In particular, compared to Z-1320, DSM 6725 grows at higher temperatures and has an expanded range of growth substrates. Moreover, the 16S rRNA sequence of strain DSM 6725 falls within the Caldicellulosiruptor clade. We therefore reclassify 'Anaerocellum thermophilum' as Caldicellulosiruptor bescii sp. nov. (type strain DSM 6725T=ATCC BAA-1888T). C. bescii strain DSM 6725T is the most thermophilic cellulose-degrading organism known. It grows up to 90 degrees C (pH 7.2) and degrades crystalline cellulose and xylan as well as untreated plant biomass, including potential bioenergy plants such as poplar and switchgrass.