Homoacetogens are a group of bacteria capable of converting synthetic gases(H2/CO2 and/or CO)to acetate,ethanol,and other chemicals via acetyl-CoA pathway.We studied homoacetogens population dynamics in waste activate sludge,freshwater sediment,anaerobic methanogenic sludge,and cow manure during enrichments using H2/CO2 and formate as substrates.Enrichment of homoacetogenic populations coincided with accumulations of short chain fatty acids such as acetate and butyrate.