A moderately thermophilic, nitrate-reducing bacterium, strain Yu37-1(T), was isolated from hot spring water from Yumata, Nagano, Japan. Cells of strain Yu37-1(T) were strictly anaerobic, Gram-negative, non-sporulating, motile by means of a single polar flagellum, vibrio-shaped and 1.4-2.0 microm long. The temperature and pH for optimum growth were 55 degrees C and pH 7.0-7.5, respectively. Strain Yu37-1(T) grew best in basal medium without the addition of NaCl. Acetate, pyruvate, lactate, fumarate, succinate, malate, yeast extract, peptone and Casamino acids were utilized as electron donors, with nitrate as the only electron acceptor. Ammonium was the end product from nitrate. The G+C content of the genomic DNA was 35.1 mol%. Phylogenetic analysis based on the 16S rRNA gene revealed that strain Yu37-1(T) could be accommodated in the family Deferribacteraceae and that its closest neighbours were members of the five genera of the family Deferribacteraceae, namely Deferribacter, Denitrovibrio, Flexistipes, Geovibrio and Mucispirillum, with similarities of only 83.2-86.2 %. The growth temperature and salinity range for growth of strain Yu37-1(T) differed from those of the phylogenetically related organisms. On the basis of phenotypic features and phylogenetic position, a novel genus and species are proposed, Calditerrivibrio nitroreducens gen. nov., sp. nov. Strain Yu37-1(T) (=NBRC 101217(T) =DSM 19672(T)) is the type strain of Calditerrivibrio nitroreducens.