A spherical thermoacidophilic archaeon, strain TA-2, was obtained from acidic hot springs located in Ohwaku Valley, Hakone, Japan. This isolate is an obligate aerobic chemoorganoheterotroph that grows optimally at about 75 degrees C, pH 2.8. The G + C content of DNA from TA-2 is 47 mol%. The 16S rRNA gene from TA-2 showed more than 99% similarity with those of Metallosphaera sedula and Metallosphaera prunae and less than 92% similarity with other members of the order Sulfolobales. DNA-DNA hybridization experiments showed more than 93% genomic DNA homology among TA-2, M. sedula DSM5348T, and M. prunae DSM10039T. However, TA-2 lacks calditoglycerocaldarchaeol derivatives, which are usually found in the membrane lipids of members of the order Sulfolobales. Therefore, calditoglycerocaldarchaeol may not be essential for survival in thermophilic and acidophilic environments. The isolate was deposited as Metallosphaera sedula TA-2 (JCM 9064, IFO 15160).