Crenarchaeota is one of the four phyla of Archaea. Crenarchaeota comprises both hyperthermophilic and cold-dwelling prokaryotes. The hyperthermophilic species of Crenarchaeota tend to cluster closely together and occupy short branches on the 16S rRNA gene phylogenetic tree. These organisms are considered good models for early Archaea. Phylogenetically, these are a more rapidly evolving species. Most hyperthermophilic Crenarchaeota have been isolated from geothermal heated soils, waters containing elemental sulfur and sulfides, or hydrothermal vents. Among the hyperthermophilic Crenarchaeota, we can find members of the Sulfolobales, Thermoproteales, and Desulfurococcales orders. One species of Sulfolobus, S. acidocaldarius, was the first hyperthermophilic Archaea discovered. It was isolated by Thomas Brock and colleagues in Yellowstone National Park, U.S.A., in 1970. Special mention should be given to Pyrodyctum fumarii that can grow at 113 °C.