Abstract: A methane fermentation process wherein a strain of the genus Methanobacterium is cultivated in a fermenter containing at least a carbon source, a nitrogen source, inorganic salts and micro-flora for methane fermentation. The strain has a methanogenic activity above 8.7.times.10.sup.-8 ml/cell.day in the stationary phase of growth, exhibits the methanogenic activity even at an oxygen partial pressure of 1/30 atm. and is allowed to live under an oxygen partial pressure up to 1/5 atm. The seed of the strain is obtained by anaerobically cultivating the strain in a culture medium containing at least nitrogen sources, inorganic salts and reducing agents in a mesophilic range of 25.degree. to 45.degree. C. using a substrate such as a mixed gas of hydrogen and carbon dioxide, a formate or an acetate. The growth of the strain is facilitated when cocultured with a strain belonging to the genus Clostridium.