Bacterial community composition was investigated in a biocathode of an external-powered bio-electrochemical denitrifying system (BEDS), by analyzing 16S rRNA gene sequences determined from a high-throughput 454 pyrosequencing technology. The bacterial community of the inoculum sludge was mainly composed of Proteobacteria, Bacteroidetes, and Acidobacteria. Over 45 days of the BEDS, Bacteroidetes and Firmicutes were enriched on the biocathode. Putative denitrifying Clostridia belonging to the Firmicutes were distinctively detected on the biocathode via phylogenetic analysis and appeared to be ubiquitous in various autotrophic, heterotrophic, and BEDSs. Elucidation of the bacterial community composition in the denitrification system deepens our knowledge about the bacteria involved in bio-electrochemical denitrification. (C) 2012 Elsevier Ltd. All rights reserved.