The aerobic and anaerobic biodegradations of surfactants were examined by bacteria from sewage sediments. During the biodegradations, CO₂ under aerobic conditions, or CO₂ and CH₄ under anaerobic conditions were determined as the final products of the carbon sources. The relation of structure and biodegradability with the degree of biodegradation was examined. Based on the results obtained the surfactants were classified into three groups : (A) sodium alkyl sulfate (AS), sodium salt of fatty acid (Soap) and alkyl poly (oxyethylene) ether (AE), all biodegradable under both aerobic and anaerobic conditions ; (B) sodium α-olefinsulfonate (AOS), sodium linear alkylbenzenesulfonate (LAS) and sodium alkylpoly (oxyethylene) sulfate (AES), all biodegradable under aerobic conditions but less so under anaerobic conditions ; (C) p-alkylphenylpoly (oxyethylene) ether (APE) and sodium alkylphenylpoly (oxyethylene) sulfate (APES). These were extremely resistant to both the aerobic and anaerobic biodegradations.