A phylogenetic analysis of all validly published members of the Crenarchaeota, including several new isolates from our laboratory, suggests three orders within this archaeal kingdom. The Thermoproteales consist of both the rod-shaped, hyperthermophilic, neutrophilic representatives of the Thermoproteaceae and the members of the new family Thermofilaceae. The Sulfolobales harbor all thermoacidophilic, coccoid organisms. The neutrophilic, hyperthermophilic cocci are members of a new order tentatively named "Igneococcales." This order comprises two families, the Desulfurococcaceae, characterized by maximal growth temperature of up to 100 degrees C, and the new family Pyrodictiaceae, for which optimal growth occurs at temperatures above 100 degrees C.