From laboratory anaerobic bioreactor operating under seawater condition (26 g NaCl-L-1), a methanogenic archaeon strain M37 was isolated via dilution series in anaerobic agar tubes containing artificial seawater medium with a mixture of acetate, methanol and trimethylamine (10 mM of each) as substrates. The isolate showed the highest homology in the 16S rDNA sequence with Methanosarcina siciliae (98 percent) and was able to produce methane with acetate but not with H2/CO2 or other tested substrates such as methanol and trimethylamine. Especially, strain M37 grew best in the medium containing 17 - 40 g. L-1 NaCl, corresponding to brackish and seawater conditions in nature. Besides that, strain M37 grew in a broad range of temperature and pH values, where the optimal conditions were 37°C and pH 7.5. In comparison to three reported strains of Methanosarcina siciliae, the strain M37 showed the highest similarity in the physiological properties to strain C2J which was isolated from marine sediment and described as a moderate halophilic methanogen. Owning such physiological properties, strain M37 could be used as methanogenic source for adding into anaerobic digesters that operate under high salt concentration conditions to support the degradation process of organic carbon there.