A novel thermophilic, anaerobic bacterium, strain F1F22 , was isolated from hot spring water collected in northern Tunisia. The cells were non-motile, Gram-negative and helical with hooked ends, 0.5×10-32 µm in size. Growth of the strain was observed at 45-70 °C (optimum, 55 °C), in 0.0-1.0 % (w/v) NaCl (optimum without NaCl) and at pH 6.5-8.5 (optimum, pH 7.5). Yeast extract was required for growth, and the strain grew on glucose, sucrose and maltose. The major fatty acids were C (40.2 %), iso-C (30.2 %) and C DMA (14.5 %). The genome consisted of a circular chromosome (2.5 Mb) containing 2672 predicted protein-encoding genes with a G+C content of 43.15 mol %. Based on a comparative 16S rRNA gene sequence analysis, strain F1F22 formed a deeply branching lineage within the phylum , class , order , and had only low sequence similarity to other species of the phylum (lower than 83 %). Genome-based analysis of average nucleotide identity and digital DNA-DNA hybridization of strain F1F22 with DSM 7334 , ATCC 43811 and DSM 6578 showed values between 63.26 and 63.52 %, and between 20 and 25 %. Hence, we propose strain F1F22 as a representative of a novel family ( fam. nov.), genus and species of : gen. nov., sp. nov. (type strain F1F22 =JCM 31314 =DSM 101182 ).