A new thermoacidophilic, obligately chemolithotrophic, facultatively aerobic archaeon Acidianus S5(T), was isolated from a Tengchong acidothermal spring in southwestern China. It is a Gram-negative, nonmotile, irregular coccoid organism with a cell diameter of 1.2 microm. The optimal pH and temperature for growth are 2.5 and 70 degrees C, respectively. Under anaerobic conditions, the organism reduces elemental sulfur with molecular hydrogen, producing hydrogen sulfide. Under aerobic conditions, it oxidizes elemental sulfur and produces sulfuric acid. No growth occurs when it is cultivated in an iron medium, indicating that ferrous iron cannot serve as an energy source. The G+C content is 38% (mol/mol), which is much different from that of other Acidianus species (31%-32.7%). The phylogenetic distances, based on 16S rDNA sequences, to A. brierleyi, A. infernus, and A. ambivalens were 0.2, 2.6, and 2.5%, respectively. DNA-DNA hybridization rates of strain S5(T) to A. brierleyi, A. infernus, and A. ambivalens are 44, 22, and 23%, respectively. Thus, a new name, Acidianus tengchongensis sp. nov., is proposed for this strain S5(T).