Methanococcus jannaschii, an extremely thermophilic methanogen isolated from a deep-sea hydrothermal vent, was grown at 80'C in continuous culture on a mineral salts medium gassed with H2 and COZ at three different flow rates. The maximum specific growth rate was 0.56 h-', and the maximum specific methane productivity was 0.32 (mol g-' h-'). Uncoupling of growth and methane production was evidenced by an increase in the non-growth-associated rate of methane formation, p, with increasing gaseous input.The specific hydrogenase activity exhibited growthassociated behavior at low growth rates, but showed no dependence on growth at higher growth rates. The growth dependence of hydrogenase activity is consistent with the pressure dependence of hydrogenase activity measured in previous experiments. In contrast, the specific protease activity was independent of the growth rate over the entire range of dilution rates studied.