Methanococcus thermolithotrophicus was grown in a mineral salts medium at 65° C in a fermenter gassed with H2 and CO2, which were the sole carbon and energy sources. Evolution of growth parameters during batch culture experiments showed the existence of an uncoupling phenomenon. The growth was then studied using a continuous technique and steady states for various gas flow rates were obtained. Y CH 4and the maintenance coefficient varied with the gas input. The maximum Y CH 4 determined for Methanococcus thermolithotrophicus was 3.33 g·mol-1 CH4. An excess of energy and carbon sources induced uncoupling of growth.