<jats:p>An aerobic, saccharolytic, obligately thermophilic, motile, non-spore-forming bacterium, strain T49<jats:sup>T</jats:sup>, was isolated from geothermally heated soil at Hell’s Gate, Tikitere, New Zealand. On the basis of 16S rRNA gene sequence similarity, T49<jats:sup>T</jats:sup> is the first representative of a new class in the newly described phylum <jats:italic>Armatimonadetes</jats:italic>, formerly known as candidate division OP10. Cells of strain T49<jats:sup>T</jats:sup> stained Gram-negative and were catalase-positive and oxidase-negative. Cells possessed a highly corrugated outer membrane. The major fatty acids were 16 : 0, i17 : 0 and ai17 : 0. The G+C content of the genomic DNA was 54.6 mol%. Strain T49<jats:sup>T</jats:sup> grew at 50–73 °C with an optimum temperature of 68 °C, and at pH 4.7–5.8 with an optimum growth pH of 5.3. A growth rate of 0.012 h<jats:sup>−1</jats:sup> was observed under optimal temperature and pH conditions. The primary respiratory quinone was MK-8. Optimal growth was achieved in the absence of NaCl, although growth was observed at NaCl concentrations as high as 2 % (w/v). Strain T49<jats:sup>T</jats:sup> was able to utilize mono- and disaccharides such as cellobiose, lactose, mannose and glucose, as well as branched or amorphous polysaccharides such as starch, CM-cellulose, xylan and glycogen, but not highly linear polysaccharides such as crystalline cellulose or cotton. On the basis of its phylogenetic position and phenotypic characteristics, we propose that strain T49<jats:sup>T</jats:sup> represents a novel bacterial genus and species within the new class <jats:italic>Chthonomonadetes</jats:italic> classis nov. of the phylum <jats:italic>Armatimonadetes</jats:italic>. The type strain of <jats:italic>Chthonomonas calidirosea</jats:italic> gen. nov., sp. nov. is T49<jats:sup>T</jats:sup> ( = DSM 23976<jats:sup>T</jats:sup> = ICMP 18418<jats:sup>T</jats:sup>).</jats:p>