A novel aerobic hyperthermophilic archaeon was isolated from a coastal solfataric vent at Kodakara-Jima Island, Japan. The new isolate, strain K1, is the first strictly aerobic organism growing at temperatures up to 100 degrees C. It grows optimally at 90 to 95 degrees C, pH 7.0, and a salinity of 3.5%. The cells are spherical shaped and 0.8 to 1.2 microns in diameter. Various proteinaceous complex compounds served as substrates during aerobic growth. Thiosulfate stimulates growth without producing H2S. The core lipids consist solely of C25-isopranyl archaeol (glycerol diether). The G + C content of the genomic DNA is 67 mol%. Phylogenetic analysis based on 16S rRNA sequence indicates that strain K1 is a new member of Crenarchaeota. On the basis of our results, the name Aeropyrum pernix gen. nov., sp. nov. is proposed (type strain: K1; JCM 9820).