A novel alkalitolerant, anaerobic bacterium, designated strain sk.kt5(T), was isolated from a metal coupon retrieved from a corrosion-monitoring reactor of a Danish district heating plant (Skanderborg, Jutland). The cells of strain sk.kt5(T) were motile, rod-shaped (0.4-0.6 x 2.5-9.6 microm), stained Gram-positive and formed endospores. Strain sk.kt5(T) grew at pH 7.6-10.5 (with optimum growth at pH 8.0-9.5), at temperatures in the range 23-44 degrees C (with optimum growth at 35-37 degrees C), at NaCl concentrations in the range 0-5 % (w/v) (with optimum growth at 0-0.5 %) and required yeast extract for growth. Only a limited number of substrates were utilized as electron donors, including betaine, formate, lactate, methanol, choline and pyruvate. Elemental sulfur, sulfite, thiosulfate, nitrate and nitrite, but not sulfate or Fe(III) citrate, were used as electron acceptors. The G+C content of the DNA was 41.6 mol%. Phylogenetic analyses of the sequence data for the dsrAB genes [encoding the major subunits of dissimilatory (bi)sulfite reductase] and the 16S rRNA gene placed strain sk.kt5(T) within a novel lineage in the class Clostridia of the phylum Firmicutes. Taken together, the physiological and genotypic data suggest that strain sk.kt5(T) represents a novel species within a novel genus, for which the name Desulfitibacter alkalitolerans gen. nov., sp. nov. is proposed. The type strain of Desulfitibacter alkalitolerans is sk.kt5(T) (=JCM 12761(T)=DSM 16504(T)).