The objective of this work was to evaluate methanogenesis in relation to the changes in performance and microbial diversity of cellulose-fed microbial fuel cells (MFCs). Replicate MFCs were inoculated with a ruminal microbial consortium and operated under 20 (R-20 Omega) or 100 Omega (R-100 Omega) external resistances. During the first week of operation, 0.31 and 0.44 mmol l(-1) of methane were produced in the R-20 Omega and R-100 Omega MFCs, respectively. Methanogenesis was, however, suppressed to undetectable levels within 90 days of operation, accompanied with increased current production and improved coulombic efficiency. Suppressed methanogenesis coincided with changes in the concentrations of short chain fatty acids and a decrease in the microbial diversity. The results demonstrated that methanogenesis was active during the early stage of cellulose-fed MFCs but this activity declined over prolonged operation. (C) 2012 Elsevier Ltd. All rights reserved.