Electricity can be directly generated by bacteria in microbial fuel cells (MFCs) from a variety of biodegradable substrates, including cellulose. Particulate materials have not been extensively examined for power generation in MFCs, but in general power densities are lower than those produced with soluble substrates under similar conditions likely as a result of slow hydrolysis rates of the particles. Cellulases are used to achieve rapid conversion of cellulose to sugar for ethanol production, but these enzymes have not been previously tested for their effectiveness in MFCs. It was not known if cellulases would remain active in an MFC in the presence of exoelectrogenic bacteria or if enzymes might hinder power production by adversely affecting the bacteria. Electricity generation from cellulose was therefore examined in two-chamber MFCs in the presence and absence of cellulases. The maximum power density with enzymes and cellulose was 100 ± 7 mW/m2 (0.6 ± 0.04 W/m3), compared to only 12 ± 0.6 mW/m2 (0.06 ± 0.003 W/m3) in the absence of the enzymes. This power density was comparable to that achieved in the same system using glucose (102 ± 7 mW/m2, 0.56 ± 0.038 W/m3) suggesting that the enzyme successfully hydrolyzed cellulose and did not otherwise inhibit electricity production by the bacteria. The addition of the enzyme doubled the Coulombic efficiency (CE) to CE = 51% and increased COD removal to 73%, likely as a result of rapid hydrolysis of cellulose in the reactor and biodegradation of the enzyme. These results demonstrate that cellulases do not adversely affect exoelectrogenic bacteria that produce power in an MFC, and that the use of these enzymes can increase power densities and reactor performance. Biotechnol. Bioeng. © 2008 Wiley Periodicals, Inc.