Three spherical thermoacidophilic archaea (strains TA-1T, TA-13, TA-14) were obtained from acidic hot springs located in Ohwaku Valley, Hakone, Japan. All the isolates are facultatively anaerobic, and grew optimally at around 85 degrees C, pH 2.0. Isolate TA-1T was characterized further. The G + C content of DNA from TA-1T is 33 mol%. Although these properties resemble those of the genus Acidianus, the sequence of the 16S rRNA gene from strain TA-1T was more similar to that of species of Stygiolobus than of Acidianus. DNA-DNA hybridization experiments also indicated that strain TA-1T is clearly distinguished phylogenetically from the members of Acidianus, Sulfolobus and Metallosphaera. On the basis of the distinct physiological and molecular properties, we describe the new strains as members of the new genus Sulfurisphaera. The type species of the genus is Sulfurisphaera ohwakuensis, and the type strain of the species is TA-1T (= IFO 15161T).