A mesophilic, hydrogenotrophic methanogen, designated strain Ki8-1(T), was isolated from soil. Cells were strictly anaerobic, Gram-stain-negative, non-sporulating, motile by means of a single flagellum or tufted flagella, and curved or wavy rod-shaped (11-25 μm long). The temperature and pH for optimum growth were 30 °C and 7.5. The strain grew best in basal medium without the addition of NaCl. Methane was produced from H(2) and formate. Acetate or yeast extract was required for growth. The G+C content of the genomic DNA of strain Ki8-1(T) was 45.3 mol%. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain Ki8-1(T) was a member of the genus Methanospirillum and showed 95.1% sequence similarity to Methanospirillum hungatei NBRC 100397(T). On the basis of its phenotypic characteristics and phylogenetic position, strain Ki8-1(T) is considered to represent a novel species of the genus Methanospirillum, for which the name Methanospirillum lacunae sp. nov. is proposed. The type strain is Ki8-1(T) (NBRC 104920(T) =JCM 16384(T) =DSM 22751(T)). Emended descriptions of the genus Methanospirillum and of Methanospirillum hungatei are also provided.