A mesophilic, non-motile, hydrogenotrophic, rod-shaped methanogen, designated M2(T), was isolated from Holocene permafrost sediments of the Kolyma lowland in the Russian Arctic. Cells were 3-6 μm long and 0.45-0.5 μm wide. Strain M2(T) grew on H(2)/CO(2) and formate. Optimum conditions for growth were 37°C, pH 6.8-7.2 and 0.1 M NaCl. The DNA G+C content was 38.1 mol%. On the basis of 16S rRNA gene sequence comparison with known methanogens, strain M2(T) was affiliated with the genus Methanobacterium and was most closely related to Methanobacterium veterum MK4(T) and Methanobacterium bryantii DSM 863(T) (both 99 % 16S rRNA gene sequence similarity). However, no significant DNA-DNA relatedness was observed between strain M2(T) and these type strains. We propose that strain M2(T) represents a novel species, with the name Methanobacterium arcticum sp. nov., with type strain M2(T) (=DSM 19844(T) =VKM B-2371(T)).