both growth and methanogenesis of Methanosarcina barkeri are completely inhibited by sodium dodecylbenzene sulphonate at between 15 and 20 mg·1−1. At lower concentrations growth of cultures was delayed, but no uncoupling of methanogenesis from growth was observed. Higher concentrations of detergent (50 mg·1−1) produced marked alterations in the surface structures of organisms observed in scanning electron micrographs. Thus levels of a detergent common in anaerobic sewage treatment plants can inhibit methanogenesis, the terminal stage in the anaerobic digestion process.