A bacterial isolate with an optimum growth temperature of about 55 degrees C was recovered on a medium composed of one part Kligler's iron agar and four parts of Thermus Agar from the host spring at S?o Pedro do Sul in central Portugal. Phylogenetic analyses using the 16S rRNA gene sequence of strain SPS-1037T indicated that the new organism represented a new genus and species of beta-Proteobacteria. The major fatty acids of strain SPS-1037T are C16:0 and C17:0. Ubiquinone 8 is the major respiratory quinone, and the major polar lipids are phosphatidylethanolamine and phosphatidylglycerol. The new isolate is aerobic and chemolithoheterotrophic. Thiosulfate and tetrathionate were oxidized to sulfate. The growth yield of the organism was improved by the addition of thiosulfate to media containing organic carbon sources, but the organism did not grow autotrophically under the conditions examined. Heterotrophic growth of strain SPS-1037T occurs on amino acids and organic acids, but this organism does not assimilate carbohydrates. On the basis of the phylogenetic analyses, and physiological and biochemical characteristics, it is proposed that strain SPS-1037T represents a new genus and a new species for which the name Tepidimonas ignava is proposed.