A hydrogen-consuming methanogenic enrichment showed complete reduction of ethene to ethane. The ethene reduction was coupled to the formation of methane and addition of bromoethane sulfonate completely inhibited the reduction of ethene. At an ethene concentration of 0.8% in the gas phase (approximately 90 Wmol l 3I in the liquid phase), the formation of ethane from ethene was completely inhibited and methane formation was inhibited by 85%. Three dominant microorganisms were present in the enrichment. A highly motile, oval-shaped homoacetogenic bacterium, and two long, rod-shaped methanogens. One methanogen was irregularly crooked and the other one was Methanospirillum-like. The acetogen and the irregularly crooked methanogen were isolated, but in pure culture they did not reduce ethene. So far, we have been unable to isolate the other methanogen. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.