Abstract: Crenarchaeota are of the three Kingdoms in Archaea with the other two Kingdoms being Euryarchaeota and Korarchaeota．While Euryarchaeota have long been recognized as an important group of the prokaryotes and have been well studied，Crenarchaeota and Korarchaeota have received increasing attention only recently．This review focuses on Crenarchaeota because recent advances in ecological and evolutionary studies of Archaea have mostly occurred in this kingdom．One important observation is that all isolated species of Crenarcheota are extremely thermophilic．whereas non-thermophilic Crenarchaeota have so far resisted attempts of isolation．Thermophilic Crenarchaeota are mostly found in deep sea hydrothermal vents and terrestrial hot springs，which are characterized by growth at high temperature(>80℃)and low pH(<6)．A variety of thermophilic Crenarchaeota can use reduced inorganic chemicals such as H2，and reduced sulfur for chemoautotrophic growth．This trait，along with the group’s deep branching in the tree of life，has lead to the belief that the last common ancestor of life may be a thermophilic autotroph growing in a hydrothermal system．Non-thermophilic Crenarchaeota have a very close phylogenetic relationship to the thermophilic Crenarchaeota． Although currently unculturable， non-thermophilic Crenarchaeota have been increasingly recovered from low-to moderate-temperature environments, which range from open marine to terrestrial soils．1akes．and subsurface．This is largely due to the development of the culture．independent molecular techniques，such as sequencing of the 16S rRNA genes．Furthermore，studies of non-thermophilic Crenarchaeota in oceans and soil suggest that certain members of the non-thermophilic Crenarchaeota may have very important roles in the biogeochemical cycling of carbon．