Methane emission was inhibited by aluminium ion in paddy fields. Addition of Al3+ (20 mM) to the culture medium containing cells of pure Methanosarcina barkeri, inhibited methanogenesis. Methanogenic co-factor, F-420, was isolated and purified from Methanosarcina barkeri MS. Spectrophotometric and spectrofluorometric analysis of interaction between co-factor, F-420, and Al3+ revealed that they formed a complex compound that might have blocked methanogenesis.