A novel moderately thermophilic, anaerobic, heterotrophic bacterium (strain SY095(T)) was isolated from a hydrothermal vent chimney located on the Southwest Indian Ridge at a depth of 2730 m. Cells were Gram-stain-positive, motile, straight to slightly curved rods forming terminal endospores. SY095(T) was grown at 45-60 degrees C (optimum 50-55 degrees C), pH 6.0-7.5 (optimum 7.0), and in a salinity of 1-4.5% (w/v) NaCl (optimum 2.5%). Substrates utilized by SY095(T) included fructose, glucose, maltose, N-acetyl glucosamine and tryptone. Casamino acid and amino acids (glutamate, glutamine, lysine, methionine, serine and histidine) were also utilized. The main end products from glucose fermentation were acetate, H-2 and CO2. Elemental sulphur, sulphate, thiosulphate, sulphite, fumarate, nitrate, nitrite and Fe(III) were not used as terminal electron acceptors. The predominant cellular fatty acids were C-14:0 (60.5%) and C-16:0 (7.6%). The main polar lipids consisted of diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, five unidentified phospholipids and two unidentified aminophospholipids. No respiratory quinones were detected. The chromosomal DNA G+C content was 30.8 mol%. The results of phylogenetic analysis of the 16S rRNA gene sequences indicated that SY095(T) was closely related to Crassaminicella profunda Ra1766H(T) (95.8% 16S rRNA gene sequence identity). SY095(T) exhibited 78.1% average nucleotide identity (ANI) to C. profunda Ra1766H(T). The in silico DNA-DNA hybridization (DDH) value indicated that SY095(T) shared 22.7% DNA relatedness with C. profunda Ra1766H(T). On the basis of its phenotypic, genotypic and phylogenetic characteristics, SY095(T) is suggested to represent a novel species of the genus Crassaminicella, for which the name Crassaminicella thermophila sp. nov. is proposed. The type strain is SY095(T) (=JCM 34213=MCCC 1K04191). An emended description of the genus Crassaminicella is also proposed.