Strain KhalAKB1 T , a mesophilic, anaerobic, rod-shaped bacterium, was isolated from water collected from a mesothermic Tunisian spring. Cells were Gram-staining-positive rods, occurring singly or in pairs and motile by one lateral flagellum. Strain KhalAKB1 T grew at 15–45 °C (optimum 30 °C), at pH 5.5–8.5 (optimum pH 7.0) and in the presence of 0–35 g NaCl l − 1 (optimum 1 g NaCl l − 1 ). It fermented yeast extract and a wide range of carbohydrates including cellobiose, d -glucose, d -ribose, sucrose, d -xylose, maltose, d -galactose and starch as electron donors. Acetate, ethanol, CO 2 and H 2 were end products of glucose metabolism. It reduced elemental sulfur, but not sulfate, thiosulfate or sulfite, into sulfide. The DNA G+C content was 37.6 mol%. The predominant cellular fatty acids were C 14 : 0 and C 16 : 0 . Phylogenetic analysis of the 16S rRNA gene sequence suggested Fusibacter bizertensis as the closest relative of this isolate (identity of 97.2 % to the type strain). Based on phenotypic, phylogenetic and genotypic taxonomic characteristics, strain KhalAKB1 T is proposed to be assigned to a novel species within the genus Fusibacter , order Clostridiales , Fusibacter fontis sp. nov. The type strain is KhalAKB1 T ( = DSM 28450 T = JCM 19912 T ).