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 mm long and 0.45-0.5 mm wide. Strain M2 T grew on H 2 /CO 2 and formate. Optimum conditions for growth were 37 6C, 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 (5DSM 19844 T 5VKM B-2371 T ).At the time of writing, the genus Methanobacterium comprises 24 species with validly published names, although a number of them have been reclassified (http://www. bacterio.cict.fr/m/methanobacterium.html). The remaining members of the genus are hydrogenotrophic mesophiles, and only Methanobacterium subterraneum (Kotelnikova et al., 1998) and Methanobacterium aarhusense (Shlimon et al., 2004) were isolated from permanently cold habitats.One of the reasons for the presence of methane in permafrost is methane formation in sediments at above-zero temperatures followed by its conservation during freezing. At the same time, one cannot exclude the possibility of methane formation in permafrost at sub-zero temperatures. Previously, we isolated three strains of methanogenic archaea from samples of Pliocene and Holocene permafrost from Eastern Siberia (Rivkina et al., 2007). Strains M2 T , MK3 and MK4 T were non-motile hydrogenotrophic rods. Strain MK4 T has been assigned to a novel species, Methanobacterium veterum (Krivushin et al., 2010). In the present study, we report the characterization of strain M2 T .Strain M2 T was isolated from a sample of deep (2.0 m) Holocene permafrost sediments of the Kolyma lowland, Russia (70 u 069 N 154 u 049 E), by long-term enrichment (Rivkina et al., 2007). The protocols for drilling and subsequent handling of cores ensured that the retrieved material remained uncontaminated (Shi et al., 1997). Purity of the culture of strain M2 T was confirmed using the chromosomal DNA restriction pattern. The polyphasic characterization used the minimal standards detailed by Boone & Whitman (1988). M. veterum MK4 T and Methanobacterium bryantii VKM B-1629 T were used as reference strains.Gram-staining was performed following standard protocols (Smibert & Krieg, 1994) and cells of strain M2 T stained Gram-negative. Cell morphology was examined using phase-contrast microscopy (I-2; Lumam) at 61350 magnification and electron microscopy (JEM-100; JEOL) with ultrathin sections as described previously (Shcherbakova et al., 2005). Cells of strain M2 T were non-motile, slightly curved rods (Fig. 1a), 0.45-0.50...