Deep subterranean granitic aquifers have not been explored regarding methanogens until now. Three autotrophic methane-producing Archaea were isolated from deep granitic groundwater at depths of 68, 409 and 420 m. These organisms were non-motile, small, thin rods, 0.1-0.15 micron in diameter, and they could use hydrogen and carbon dioxide or formate as substrates for growth and methanogenesis. One of the isolates, denoted A8p, was studied in detail. It grew with a doubling time of 2.5 h under optimal conditions (20-40 degrees C, pH 7.8-8.8 and 0.2-1.2 M NaCl). Strain A8p is eurythermic as it grew between 3.6 and 45 degrees C. It was resistant to up to 20 mg bacitracin l-1. The G + C content was 54.5 mol%, as determined by thermal denaturation. Phylogenetic studies based upon 16S rRNA gene sequence comparisons placed the isolate A8p in the genus Methanobacterium. Phenotypic and phylogenetic characters indicate that the alkaliphilic, halotolerant strain A8p represents a new species. We propose the name Methanobacterium subterraneum for this species, and strain A8p (= DSM 11074T) is the type strain.