An anaerobic, thermophilic, filamentous (0.45 × >100 µm) bacterium, designated D1-25-10-4(T), was isolated from a deep hot aquifer in France. Cells were non-motile and Gram-negative. Growth was observed at 43-65 °C (optimum 55 °C), at pH 6.8-7.8 (optimum pH 7.0) and with 0-5 g NaCl l(-1) (optimum 0 g NaCl l(-1)). Strain D1-25-10-4(T) was a chemo-organotroph and fermented ribose, maltose, glucose, galactose, arabinose, fructose, mannose, sucrose, raffinose, xylose, glycerol, fumarate, peptone, starch and xylan. Yeast extract was required for growth. Sulfate, thiosulfate, sulfite, elemental sulfur, nitrate, nitrite and fumarate were not used as terminal electron acceptors. The G+C content of the DNA was 61.9 mol%. The major cellular fatty acids of strain D1-25-10-4(T) were C(17 : 0), C(18 : 0,) C(16 : 0) and iso-C(17 : 0). The closest phylogenetic relative of strain D1-25-10-4(T) was Caldilinea aerophila STL-6-O1(T) (97.9 % 16S rRNA gene sequence similarity). DNA-DNA relatedness between strain D1-25-10-4(T) and Caldilinea aerophila DSM 14535(T) was 8.7 ± 1 %. On the basis of phylogenetic, genotypic and phenotypic characteristics, strain D1-25-10-4(T) represents a novel species within the genus Caldilinea, class Caldilineae, phylum Chloroflexi, for which the name Caldilinea tarbellica sp. nov. is proposed. The type strain is D1-25-10-4(T) ( = DSM 22659(T) = JCM 16120(T)).