An anaerobic, thermophilic bacterium, strain SET IS-9(T), was isolated from an Icelandic hot spring. Cells of strain SET IS-9(T) are short, slightly curved, motile rods. The strain grows chemolithotrophically on CO, producing equimolar quantities of H(2) and CO(2). It also grows fermentatively on lactate or pyruvate in the presence of yeast extract (0.2 g l(-1)). Products of pyruvate fermentation are acetate, CO(2) and H(2). Growth occurs at 50-70 °C, with an optimum at 65 °C, and at pH 5.0-8.0, with an optimum at pH 5.5-6.0. The generation time during chemolithotrophic growth on CO under optimal conditions is 2.0 h. 16S rRNA gene sequence analysis suggested that the organism belongs to the genus Carboxydothermus. On the basis of phenotypic features and phylogenetic analysis, Carboxydothermus islandicus sp. nov. is proposed, with the type strain SET IS-9(T) ( = DSM 21830(T) = VKM B-2561(T)). An emended description of the genus Carboxydothermus is also given.