Archaebacteria are also known as ancient bacteria. They are the oldest prokaryotes. Their characteristic feature is the absence of peptidoglycan in the cell wall and presence of proteins and polysaccharides. Some of them live under hostile conditions. They may be obligate or facultative anaerobe. They are also sub divided into methanogens, halophiles and thermoacidophiles. Methanogens are autotrophic bacteria which occur in marshy areas. They form methane with the help of carbon bi oxide, formic acid and hydrogen. Some of them occur as symbiotic and live inside stomach of herbivorous animals. Halophiles are known as salt resistant bacteria. They are heterotrophic in their nutrition. They are aerobic and have a pigment to protect them from sun rays. Their resistant to salt has been possible due to their high salt content, mucin covering, salt resistant lipids and no sap vacuoles. Thermoacidophiles are high temperature and acid resistant bacteria. Their ability is due to presence of special lipids in the cell membrane. They also have enzymes which can with stand extreme conditions. They can survive at very low pH that is 2 and at high temperature as 80 degree Celsius. They are chemosynthetic. They represent the earliest form of life and are referred as Living Fossils.