A new methanogenic isolate, designated as strain O1M9704 (= OCM 667), was isolated from the sediment of the estuarine environment in Eriln Shi, Taiwan. This strain grew on trimethylamine and methanol, but it did not catabolize H-2-CO2, acetate, or formate. Cells grew optimally at 37 degrees C with 0.5% NaCl in neutral pH. The cells were stained Gram-negative, nonmotile, irregular coccus 0.3-0.6 mu m in diameter. A comparison of 16S rDNA sequences phylogenetically related strain O1M9704 to Methanosarcina mazei. Gas vacuoles were observed both under phase contrast microscope and in thin sections in the electron microscope. Negative stain of electron micrographs showed a novel character of strain O1M9704 with tubule structure extended out of the cells. The tubule structure and gas vacuoles may benefit the adaptation of methanoarchaea in estuarine environment.