A new strictly anaerobic thermophilic multicellular filamentous bacterium (0.2-0.3 mu m x > 100 mu m), designated GNS-1(T), was isolated from a deep hot aquifer in France. It was non-motile, and stained Gram-negative. Optimal growth was observed at 65 degrees C, pH 7.0, and 2 g L-1 of NaCl. Strain GNS-1(T) was chemoorganotrophic fermenting ribose, glucose, galactose, arabinose, fructose, mannose, maltose, sucrose, xylose, raffinose, pyruvate, and xylan. Yeast extract was required for growth. The end products of glucose fermentation were lactate, acetate, CO2, and H-2. The G + C content of the DNA was 57.6 mol%. Its closest phylogenetic relative was Bellilinea caldifistulae with 92.5% similarity. Based on phylogenetic, genotypic and phenotypic characteristics, strain GNS-1(T) (DSM 23592(T), JCM 169801) is proposed to be assigned to a novel species of a novel genus within the class Anaerolineae (subphylum I), phylum "Chloroflexi". Thermanaerothrix daxensis gen. nov., sp. nov. The GenBank accession number is HM596746.