A methanogenic organism, designated strain HB-1(T), from the domain Archaea was isolated from groundwater sampled from a subsurface Miocene formation located in Horonobe, Hokkaido, Japan. The strain grew on methanol, dimethylamine, trimethylamine, dimethylsulfide and acetate but not on monomethylamine, H(2)/CO(2), formate, 2-propanol, 2-butanol or cyclopentanol. Cells were Gram-reaction-negative, non-motile, irregular cocci that were 1.4-2.9 µm in diameter and occurred singly or in pairs. The strain grew at 20-42 °C (optimum 37 °C), at pH 6.0-7.75 (optimum pH 7.0-7.25) and in 0-0.35 M NaCl (optimum 0.1 M). The G+C content of the genomic DNA was 41.4 mol%. 16S rRNA gene sequencing revealed that the strain was a member of the genus Methanosarcina but that it clearly differed from all recognized species of this genus (93.1-97.9 % sequence similarity). The phenotypic and phylogenetic features of strain HB-1(T) indicate that it represents a novel species of the genus Methanosarcina, for which the name Methanosarcina horonobensis sp. nov. is proposed. The type strain is HB-1(T) ( = DSM 21571(T) = JCM 15518(T) = NBRC 102577(T)).