A methanogenic archaeon, strain MK4^T, was isolated from ancient permafrost after long-term selective anaerobic cultivation. The cells were rods, 2.0–8.0 μm long and 0.40–0.45 μm wide, and stained Gram-negative. Optimal growth was observed at 28 °C and pH 7.0–7.2 and in 0.05 M NaCl. The isolate used H₂ plus CO₂, methylamine plus H₂ and methanol plus H₂ as sources for growth and methanogenesis. Phylogenetic analysis of the 16S rRNA gene sequence of the strain showed close affinity with Methanobacterium bryantii (similarity >99 % to the type strain). On the basis of the level of DNA–DNA hybridization (62 %) between strain MK4^T and Methanobacterium bryantii VKM B-1629^T and phenotypic and phylogenetic differences, strain MK4^T was assigned to a novel species of the genus Methanobacterium, Methanobacterium veterum sp. nov., with the type strain MK4^T (=DSM 19849^T =VKM B-2440^T).