A strain (strain NaT1) of a tetramethylammonium-degrading methanogen was isolated in pure culture from a sand sample obtained from Tokyo Bay. The strain degraded tetramethylammonium to methane and ammonium stoichiometrically. Trimethylammonium, dimethylammonium, methylammonium, and methanol were also degraded, but H-2/CO2, formate, and acetate were not. Cells were oval to spherical and 0.5-1.2 mu m in diameter. They occurred singly or in pairs. The strain was a mesophilic marine methanogen, and required 0.3-0.4 M Na+ for optimal growth. The growth of the strain was suppressed by nitrate, sulfate, thiosulfate, elemental sulfur, and yeast extract. Sulfite inhibited the growth completely even at 1 mM. The guanine plus cytosine content of DNA was 45 mol%. Strain NaT1 was identified as a member of the family Methanosarcinaceae.