A novel, moderately thermophilic, strictly anaerobic, mixotrophic bacterium, designated strain LF13T, was isolated from a deep-sea hydrothermal chimney sample that was collected at a vent site at 14 degrees 45' N, 44 degrees 59' W on the Mid-Atlantic Ridge. Cells were Gram-negative, thin, non-motile rods of variable length. Strain LF13T grew optimally at pH 6.8-7.0 and 60 degrees C with 2.5% (w/v) NaCl. It grew chemo-organoheterotrophically, fermenting proteinaceous substrates, pyruvate and Casamino acids. The strain was able to grow by respiration, utilizing molecular hydrogen (chemolithoheterotrophically) or acetate as electron donors and nitrate as an electron acceptor. Ammonium was formed in the course of denitrification. One-hundred milligrams of yeast extract per litre were required for growth of the strain. The G + C content of the genomic DNA of strain LF13T was 42.5 mol%. Neither 16S rDNA sequence similarity values nor phylogenetic analysis unambiguously related strain LF13T with members of any recognized bacterial phyla. On the basis of 16S rDNA sequence comparisons, and in combination with physiological and morphological traits, a novel genus, Caldithrix, is proposed, with strain LF13T (= DSM 13497T =VKM B-2286T) representing the type species, Caldithrix abyssi.