In the absence of H2, Methanococcus spp. utilized pyruvate as an electron donor for methanogenesis. For Methanococcus voltae A3, Methanococcus maripaludis JJ1, and Methanococcus vannielii, typical rates of pyruvate-dependent methanogenesis were 3.4, 2.8, and 3.9 nmol min-1 mg-1 cell dry wt, respectively. These rates were 1–4% of the rates of H2-dependent methanogenesis. For M. voltae, the concentration of pyruvate required for one-half the maximum rate of methanogenesis was 7 mM, and pyruvate-dependent methanogenesis was linear for 3 days. Radiolabeled acetate was formed from [3-14C]pyruvate, and the stoichiometry of pyruvate consumed per acetate produced was 1.12±0.27. The stoichiometry of pyruvate consumed per CH4 produced was 3.64±0.34. These values are close to the expected values of 1 acetate and 4 CH4. Although 10–30% of total cell carbon could be obtained from exogenous pyruvate during growth with H2, pyruvate did not replace the nutritional requirement for acetate in Methanococcus voltae A3 or two acetate auxotrophs of Methanococcus maripaludis, JJ6 and JJ7. These results suggest that pyruvate was not oxidized in the presence of H2. The inability to oxidize pyruvate during H2-dependent methanogenesis would prevent a futile cycle of pyruvate oxidation and biosynthesis during autotrophic growth.