Methanogenesis is the predominant terminal process of polymer degradation in anaerobic marine sediments depleted of sulfate; however, characterization of a marine consortium has not been previously reported. A marine methanogenic consortium consisting of fermentative, hydrogen-producing acetogenic and methanogenic bacteria is described. An acetotrophic methane-producing strain of bacteria was isolated. This isolate expresses three distinct morphovars: individual cocci, cell aggregates and communal cysts. Individual cocci are 1.9 µm in diameter and are nonmotile, but have fimbria-like structures. Cells have a thin protein cell wall. Acetate, methanol, methylamine, dimethylamine and trimethylamine are substrates for growth; formate and hydrogen are not. Sodium chloride and magnesium concentrations found in seawater are required for optimum growth. RNA homology values indicated that this isolate is a new species. Methanosarcina acetivorans is the proposed specific epithet. This is the first report of an acetotrophic methane-producing species indigenous to marine sediments. A method is described for mass culturing this and other acetotrophic methanogens using a pH auxostat. A strictly methylotrophic methane-producing strain was also isolated and is described. Only trimethylamine, dimethylamine, methylamine and methanol were substrates for growth. Cells were nonmotile, irregular cocci 1 µm in diameter and had a thin protein cell wall. Sodium chloride and magnesium concentrations found in seawater were required for optimum growth. Biotin was the only organic supplement required for growth in mineral medium. Fe was required for growth; Ni and Co were stimulatory. This isolate is a new genus based on RNA homology. Methanococcoides is the proposed genus and Methanococcoides methylutens is the specific epithet. Population studies suggest that this species is uniquely adapted for methylated amine degradation in marine sediments. The phylogenetic relationships of M. acetivorans, M. methylutens and other acetotrophic and methylotrophic species were determined by RNA and DNA homology techniques. Phylogenetic and physiological results in this study indicated that the population of methanogenic bacteria in marine sediments is both unique and equally diverse as the population of freshwater methanogenic species. / Ph. D.