Enrichment cultures containing marine plankton from oxygenated coastal waters (50-108% saturated) with supersaturated levels of methane (> 700% saturated) yielded a strictly anaerobic methanogenic bacterium. Nonmotile, non-spore-forming, regular to slightly irregular cocci (0.5-0.8-mu-m) were evident by phase contrast, epifluorescence, and scanning electron microscopies. The unpurified isolate required NaCl for growth, with maximal methanogenesis at 240 mM NaCl at 22-degrees-C. The optimal temperature range for growth was 22-31-degrees-C, and the optimal range for methanogenesis was 26-35-degrees-C. Mono-, di-, and trimethylated amines or methanol were substrates for methanogenesis; sodium acetate and H2:CO2 were not. The DNA base composition was 42 +/- 1% guanine plus cytosine. Serology suggested the isolate may be a new strain of Methanococcoides methylutens. Morphology, growth physiology, DNA base content, and serology are all consistent with the type description of M. methylutens, a methylotrophic methanogen isolated from submarine sediments.