A marine mesophilic, irregular coccoid methanogen, which shows close resemblance to Methanococcus sp., was isolated from the biofilm of shiphulls docked in Los Angeles harbor. Hydrogen plus carbon dioxide or formate served as substrates for methanogenesis in a mineral salt medium. The isolate did not use acetate and methanol as sole source of carbon and energy. The organism had an optimal pH range of 6.8-7.0 and a temperature optimum of 37-degrees-C. Elevated levels of sodium chloride were required for optimum growth. Optimum levels of total sulfide and magnesium chloride for growth were 1.0 mM and 10 mM respectively. The isolate used ammonia as nitrogen source. The concentration of 30 mM ammonium chloride supported maximum growth of the isolate.