A mesophilic, hydrogenotrophic methanogen, strain ML15(T), was isolated from an aquaculture fish pond near Wang-gong, Taiwan. The cells were irregular cocci, non-motile, 1.5-2.0 microm in diameter and Gram-negative. Cells of strain ML15(T) lysed easily in the presence of SDS (0.1 g l(-1)) and the S-layer protein had an M(r) of 138 800. The catabolic substrates utilized by this strain included formate and H(2)/CO(2), but not acetate, methanol, trimethylamine or secondary alcohols. Growth did not occur in minimal medium, but was observed when yeast extract and tryptone were added. Strain ML15(T) grew fastest at 37 degrees C, pH 6.6-7.0 and with 3 % NaCl. Acetate was not required for cell growth. Trace amounts of tungstate promoted cell growth. The G+C contents of DNA of Methanofollis aquaemaris N2F9704(T) and strain ML15(T) were 59.1 and 58.4 mol%, respectively. Sequence analysis of the 16S rRNA genes of strain ML15(T) and selected Methanofollis species revealed similarities of 95-97 %. Based on the data presented here, it is proposed that strain ML15(T) (=OCM 789(T)=DSM 15483(T)) represents a novel species, Methanofollis formosanus sp. nov.