A new methanogenic isolate, designated as strain N2M9705 (=OCM 668), was isolated from an aquaculture fishpond near Wang-gong, Taiwan. This strain grew on trimethylamine and methanol, but it did not catabolize H-2-CO2, acetate, or formate. The cells were stained Gram-negative, nonmotile, irregular coccus 0.6-0.8 mu m in diameter. Gas vacuoles were observed and cell aggregated to form various sizes of granules. Cells grew optimally at 32 degrees-37 degrees C with 1% NaCl. The pH range of growth was 6.2-7.4, and higher pH inhibited the cell growth. The cells grew well in minimal medium, but growth was greatly stimulated by yeast extract and peptone. A comparison of 16S rDNA sequences of this organism phylogenetically related to Methanosarcina mazei. This is the first report of methyltrophic methanogenic isolated from an aquaculture fishpond.