A novel, strictly anaerobic, hydrogenotrophic methanogen, strain E09F.3^T, was isolated from a commercial biogas plant in Germany. Cells of E09F.3^T were Gram-stain-negative, non-motile, slightly curved rods, long chains of which formed large aggregates consisting of intertwined bundles of chains. Cells utilized H₂+CO₂ and, to a lesser extent, formate as substrates for growth and methanogenesis. The optimal growth temperature was around 40 °C; maximum growth rate was obtained at pH around 7.0 with approximately 6.8 mM NaCl. The DNA G+C content of strain E09F.3^T was 39.1 mol%. Phylogenetic analyses based on 16S rRNA and mcrA gene sequences placed strain E09F.3^T within the genus Methanobacterium. On the basis of 16S rRNA gene sequence similarity, strain E09F.3^T was closely related to Methanobacterium congolense C^T but morphological, physiological and genomic characteristics indicated that strain E09F.3^Trepresents a novel species. The name Methanobacterium aggregans sp. nov. is proposed for this novel species, with strain E09F.3^T (=DSM 29428^T=JCM 30569^T) as the type strain.