Two strains of Methanosarcina (M. Barkeri strain MS, isolated from sewage sludge, and strain UBS, isolated from lake sediments) were found to have similar cellular properties and to have DNA base compositions of 44 tool percent guanosine plus cytosine. Strain MS was selected for further studies of its one-carbon metabolism. M. barkeri grew autotrophically via H 2 oxidation/CO 2 reduction. The optimum temperature for growth and methanogenesis was 37 ~ C. H2 oxidation proceeded via an F420-dependent NADP+-linked hydrogenase. A maximum specific activity of hydrodenase in cell-free extracts, using methyl viologen as electron acceptor, was 6.0 gmol rain 9 protein at 37~ and the optimum pH (9.0). M. barkeri also fermented methanol and methylamine as sole energy sources for growth. Cell yields during growth on H 2/CO z and on methanol were 6.4 and 7.2 mg cell dry weight per mmol CH 4 formed, respectively. During mixotrophic growth o n H 2 / C O 2 plus methanol, most methane was derived from methanol rather than from CO 2. Similar activities of hydrogenase were observed in cell-free extracts from Hz/COz-grown and methanol-grown cells. Methanol oxidation apparently proceeded via carrierbound intermediates, as no methylotroph-type of methanol Abbreviations.