Caldicellulosiruptor bescii is a species of thermophilic, anaerobic cellulolytic bacteria. It was isolated from a geothermally heated freshwater pool in the Valley of Geysers on the Kamchatka Peninsula in Russia in 1990[2]. The species was originally named Anaerocellum thermophilum, but reclassified in 2010[1], based on genomic data.