Archaebacteria differ from all other bacteria (which are sometimes called eubacteria). Archaebacteria are so named because biochemical evidence indicates that they evolved before the eubacteria and have not undergone significant change since then. The archaebacteria generally grow in extreme environments and have unusual lipids in their cell membranes and distinctive RNA molecules in their cytoplasm.

One group of archaebacteria are the methanogens, anaerobic bacteria found in swamps, sewage, and other areas of decomposing matter. The methanogens reduce carbon dioxide to methane gas in their metabolism. A second group are the halobacteria, a group of rods that live in high‐salt environments. These bacteria have the ability to obtain energy from light by a mechanism different from the usual process of photosynthesis. The third type of archaebacteria are the extreme thermophiles. These bacteria live at extremely high temperatures, such as in hot springs, and are associated with extreme acid environments. Like the other archaebacteria, the extreme thermophiles lack peptidoglycan in their cell walls. Many depend on sulfur in their metabolism, and many produce sulfuric acid as an end‐product.