A novel anaerobic, mesophilic, slightly halophilic sulfate-reducing bacterium, designated strain Khaled BD4 T , was isolated from waters of a Tunisian thermal spring. Cells were vibrio-shaped or sigmoids (5–7×1–1.5 µm) and occurred singly or in pairs. Strain Khaled BD4 T was Gram-stain-negative, motile and non-sporulated. It grew at 25–45 °C (optimum 37 °C), at pH 5.5–8.3 (optimum pH 7.0) and with 0.5–8 % NaCl (optimum 3 %). It required vitamins or yeast extract for growth. Sulfate, thiosulfate, sulfite and elemental sulfur served as terminal electron acceptors, but not fumarate, nitrate or nitrite. Strain Khaled BD4 T utilized H 2 in the presence of 2 mM acetate (carbon source), but also lactate, formate, pyruvate and fumarate in the presence of sulfate. Lactate was incompletely oxidized to acetate. Amongst substrates used, only pyruvate was fermented. Desulfoviridin and c -type cytochrome were present. The G+C content of the DNA was 54.6 mol%. The main fatty acids were anteiso - C 15 : 0 , iso-C 18 : 0 , iso-C 17 : 0 and iso-C 14 : 0 . Phylogenetic analysis of the 16S rRNA gene sequence indicated that strain Khaled BD4 T had Desulfovibrio giganteus DSM 4123 T (96.7 % similarity) as its closest phylogenetic relative. On the basis of 16S rRNA gene sequence comparisons together with genetic and physiological characteristics, strain Khaled BD4 T is assigned to a novel bacterial species, for which the name Desulfovibrio biadhensis sp. nov. is proposed. The type strain is Khaled BD4 T ( = DSM 28904 T = JCM 30146 T ).