Several closely related, thermophilic and cellulolytic bacterial strains, designated JKG1 T , JKG2, JKG3, JKG4 and JKG5, were isolated from a cellulolytic enrichment (corn stover) incubated in the water column of Great Boiling Spring, NV. Strain JKG1 T had cells of diameter 0.7-0.9 mm and length~2.0 mm that formed non-branched, multicellular filaments reaching .300 mm. Spores were not formed and dense liquid cultures were red. The temperature range for growth was 45-65 6C, with an optimum of 55 6C. The pH range for growth was pH 5.6-9.0, with an optimum of pH 7.5. JKG1 T grew as an aerobic heterotroph, utilizing glucose, sucrose, xylose, arabinose, cellobiose, CM-cellulose, filter paper, microcrystalline cellulose, xylan, starch, Casamino acids, tryptone, peptone, yeast extract, acetate, citrate, lactate, pyruvate and glycerol as sole carbon sources, and was not observed to photosynthesize. The cells stained Gram-negative. Phylogenetic analysis using 16S rRNA gene sequences placed the new isolates in the class Chloroflexia, but distant from other cultivated members, with the highest sequence identity of 82.5 % to Roseiflexus castenholzii. The major quinone was menaquinone-9; no ubiquinones were detected. The major cellular fatty acids (.5 %) were C 18 : 0 , anteiso-C 17 : 0 , iso-C 18 : 0 , iso-C 17 : 0 , C 16 : 0 , iso-C 16 : 0 and C 17 : 0 . The peptidoglycan amino acids were alanine, ornithine, glutamic acid, serine and asparagine. Wholecell sugars included mannose, rhamnose, glucose, galactose, ribose, arabinose and xylose. Morphological, phylogenetic and chemotaxonomic results suggest that JKG1 T is representative of a new lineage within the class Chloroflexia, which we propose to designate Kallotenue papyrolyticum gen. nov., sp. nov., Kallotenuaceae fam. nov., Kallotenuales ord. nov. The type strain of Kallotenue papyrolyticum gen. nov., sp. nov. is JKG1 T (5DSM 26889 T 5JCM 19132 T ).