An autotrophic, hydrogenotrophic methanogen, designated strain 17A1T, was isolated from the profundal sediment of the meromictic Lake Pavin, France. The cells of the novel strain, which were non-motile, Gram-staining-negative rods that measured 2–15 µm in length and 0.2–0.4 µm in width, grew as filaments. Strain 17A1T grew in a mineral medium and its growth was stimulated by the addition of yeast extract, vitamins, acetate or rumen fluid. Penicillin, vancomycin and kanamycin reduced growth but did not completely inhibit it. Growth occurred at 14–41 °C (optimum 30 °C), at pH 5.0–8.5 (optimum pH 6.5) and with 0–0.4 M NaCl (optimum 0.1 M). The novel strain utilized H2/CO2 and methanol/H2 as substrates but not formate, acetate, methylamine/H2, isobutanol or 2-propanol. Its genomic DNA G+C content was 37.0 mol%. In phylogenetic analyses based on 16S rRNA gene sequences, strain 17A1T appeared to be a member of the genus Methanobacterium , with Methanobacterium beijingense 8-2T (96.3 % sequence similarity) identified as the most closely related established species. Based on phenotypic and phylogenetic data, strain 17A1T represents a novel species of methanogen within the genus Methanobacterium , for which the name Methanobacterium lacus sp. nov. is proposed. The type strain is 17A1T ( = DSM 24406T = JCM 17760T).