Co-cultures of N. frontalis with a formate-utilizing methanogen, Methanobacterium formicicum and/or an aceticlastic methanogen, Methanosaeta concilii, were performed for methane production from cellulose. In the co-culture with M. formicicum, ca. 16 mM CH4 was produced after 7 days without accumulation of H-2 and formate. In the co-culture with M. concilii, 12 mM CH4 was produced after 17 days with decreasing acetate production. In the tri-culture of N. frontalis with M. formicicum and M. concilii, 24 mM CH4 was produced after 17 days where acetate still remained at 23 mM, but production of lactate and ethanol decreased. When a 4-times concentrated culture broth of M. concilii was inoculated in this tri-culture system in a bioreactor, 150 mM CH4 was produced after 24 days by feeding of cellulose, although 57 mM acetate still accumulated.