AbstractIn anaerobic ecosystems, acyl lipids are initially hydrolyzed by microbial lipases with the release of free fatty acids. Glycerol, galactose, choline, and other non-fatty acid components released during hydrolysis are fermented to volatile fatty acids by the fermentative bacteria. Fatty acids are not degraded further in the rumen or other parts of the digestive tract but are subjected to extensive biohydrogenation especially in the rumen. However, in environments such as sediments and waste digesters, which have long retention times, both long and short chain fatty acids areβ-oxidized to acetate by a special group of bacteria, the H2-producing syntrophs. Long chain fatty acids can also be degraded by a-oxidation. Biotransformation of bile acids, cholesterol, and steroids by intestinal microorganisms is extensive. Many rumen bacteria have specific growth requirements for fatty acids such asn-valeric,iso-valeric, 2-methylbutyric, andiso-butyric acids. Some species have requirements for C13to C18straight-chain saturated or monoenoic fatty acids for growth.