In microbial fuel cells (MFCs) bacteria generate electricity by mediating the oxidation of organic compounds and transferring the resulting electrons to an anode electrode. The objective of this study was to test the possibility of generating electricity with rumen microorganisms as biocatalysts and cellulose as the electron donor in two-compartment MFCs. The anode and cathode chambers were separated by a proton exchange membrane and graphite plates were used as electrodes. The medium in the anode chamber was inoculated with rumen microorganisms, and the catholyte in the cathode compartment was ferricyanide solution. Maximum power density reached 55 Mw/m(2) (1.5 mA, 313 mV) with cellulose as the electron donor. Cellulose hydrolysis and electrode reduction were shown to support the production of current. The electrical current was sustained for over 2 months with periodic cellulose addition. Clarified rumen fluid and a soluble carbohydrate mixture, serving as the electron donors, could also sustain power output. Denaturing gradient gel electrophoresis (DGGE) of PCR amplified 16S rRNA genes revealed that the microbial communities differed when different substrates were used in the MFCs. The anode-attached and the suspended consortia were shown to be different within the same MFC. Cloning and sequencing analysis of 16S rRNA genes indicated that the most predominant bacteria in the anode-attached consortia were related to Clostridium spp., while Comamonas spp. abounded in the suspended consortia. The results demonstrated that electricity can be generated from cellulose by exploiting rumen microorganisms as biocatalysts, but both technical and biological optimization is needed to maximize power output.