Nitrate reduction using an electrode as a direct electron donor in a biofilm-electrode reactor was investigated to treat an aqueous solution containing high concentration of nitrate. In the biofilm-electrode reactor, nitrate was reduced in the absence of organic substances by accepting electrons from the electrode. As nitrate concentration decreased, oxidation-reduction potential (ORP) gradually decreased from -80 to -260 mV. With nested PCR products from the biofilm-electrode, bacterial community was analyzed by using denaturing gradient gel electrophoresis (DGGE). DGGE analysis revealed that 10 bands were detected. Four bands (EB4, EB6, EB7, EB10) were dominant in the biofilm. The gene analysis of 16S rDNA showed that the major populations were alpha-proteobacteria, beta-proteobacteria, gamma-proteobacteria and flavobacteria. (c) 2006 Elsevier Inc. All rights reserved.