A novel methanogen, designated strain N2F9704T, was isolated from an aquaculture fish pond near Wang-gong, Taiwan. The cells were irregular cocci, non-motile, 1.2-2.0 microm in diameter and stained gram-negative. Cells of strain N2F9704T lysed easily by SDS treatment (0.1 g l(-1)) and the S-layer protein had an Mr of 137000. The catabolic substrates used included formate and H2+CO2, but not acetate, methanol, trimethylamine or secondary alcohols. The optimal growth parameters for strain N2F9704T were pH 6.5, 37 degrees C with 0.5% NaCl. Trace amounts of tungstate not only promoted growth but also extended the range of growth conditions. Analysis of the 16S rDNA sequence revealed a phylogenetic relationship to Methanofollis species and the name Methanofollis aquaemaris sp. nov. is therefore proposed for strain N2F9704T (= OCM 746T = CCRC 16166T). Additionally, the strain was infected with a novel coccus-shaped, enveloped virus with a diameter of 200 nm.