A novel aerobic, obligately mixotrophic, moderately thermophilic, thiosulfate-oxidizing bacterium, S10T, was isolated from hot-spring sediment samples collected from Atri, Bhubaneswar, India. The cells of this isolate stained Gram-negative and were strictly aerobic, non-sporulating, rod-shaped and motile with a single polar flagellum. Strain S10T was positive for oxidase and catalase activities. It was capable of utilizing thiosulfate under mixotrophic growth conditions. Mixotrophic growth was observed at pH 6.0-8.5 and 25-45 degrees C; optimum growth occurred at pH 7.5-8.0 and 30-37 degrees C. The major cellular fatty acids were C12:0 3-OH, C16:1omega7c, C16:0, C17:0 cyclo, C18:1omega7c and C19:0 cyclo omega8c. The DNA G+C content of strain S10T was 64.8 mol%. 16S rRNA gene sequence analysis indicated that the bacterium clustered within the radiation of the genus Thiomonas and showed 98.0% similarity with Thiomonas perometabolis ATCC 23370T and Thiomonas intermedia ATCC 15466T. However, DNA-DNA reassociation values of strain S10T with Thiomonas perometabolis JCM 20426T and Thiomonas intermedia JCM 20425T, its nearest phylogenetic relatives, were 46 and 39%, respectively. On the basis of phenotypic, physiological and chemotaxonomic properties, 16S rRNA gene sequence analysis and DNA-DNA reassociation studies, it is proposed that strain S10T represents a novel species of the genus Thiomonas, Thiomonas bhubaneswarensis sp. nov.; the type strain is S10T (=DSM 18181T=JCM 14806T).