A new anaerobic, thermophilic, facultatively carboxydotrophic bacterium, strain Nor1^T, was isolated from a hot spring at Norris Basin, Yellowstone National Park. Cells of strain Nor1^T were curved motile rods with a length of 2.6–3 µm, a width of about 0.5 µm and lateral flagellation. The cell wall structure was of the Gram-negative type. Strain Nor1^T was thermophilic (temperature range for growth was 40–68 °C, with an optimum at 60 °C) and neutrophilic (pH range for growth was 6.5–7.6, with an optimum at 6.8–7.0). It grew chemolithotrophically on CO (generation time, 1.15 h), producing equimolar quantities of H₂ and CO₂ according to the equation CO+H₂O→CO₂+H₂. During growth on CO in the presence of ferric citrate or amorphous ferric iron oxide, strain Nor1^T reduced ferric iron but produced H₂ and CO₂ at a ratio close to 1:1, and growth stimulation was slight. Growth on CO in the presence of sodium selenite was accompanied by precipitation of elemental selenium. Elemental sulfur, thiosulfate, sulfate and nitrate did not stimulate growth of strain Nor1^T on CO and none of these chemicals was reduced. Strain Nor1^T was able to grow on glucose, sucrose, lactose, arabinose, maltose, fructose, xylose and pyruvate, but not on cellobiose, galactose, peptone, yeast extract, lactate, acetate, formate, ethanol, methanol or sodium citrate. During glucose fermentation, acetate, H₂ and CO₂ were produced. Thiosulfate was found to enhance the growth rate and cell yield of strain Nor1^T when it was grown on glucose, sucrose or lactose; in this case, acetate, H₂S and CO₂ were produced. In the presence of thiosulfate or ferric iron, strain Nor1^T was also able to grow on yeast extract. Lactate, acetate, formate and H₂ were not utilized either in the absence or in the presence of ferric iron, thiosulfate, sulfate, sulfite, elemental sulfur or nitrate. Growth was completely inhibited by penicillin, ampicillin, streptomycin, kanamycin and neomycin. The DNA G+C content of the strain was 51.7±1 mol%. Analysis of the 16S rRNA gene sequence revealed that strain Nor1^T belongs to the Bacillus–Clostridium phylum of the Gram-positive bacteria. On the basis of the studied phenotypic and phylogenetic features, we propose that strain Nor1^T be assigned to a new genus, Thermosinus gen. nov. The type species is Thermosinus carboxydivorans sp. nov. (type strain, Nor1^T=DSM 14886^T=VKM B-2281^T).