The improvement in electricity generation during the enrichment process of a microbial consortium was analyzed using an air-cathode microbial fuel cell (MFC) repeatedly fed with acetate that was originally inoculated with sludge from an anaerobic digester. The anodic maximum current density produced by the anode biofilm increased from 0.12 mA/cm(2) at day 28 to 1.12 mA/cm(2) at day 105. However, the microbial cell density on the carbon cloth anode increased only three times throughout this same time period from 0.21 to 0.69 mg protein/cm(2), indicating that the biocatalytic activity of the consortium was also enhanced. The microbial activity was calculated to have a per biomass anode-reducing rate of 374 mu mol electron g protein(-1) min(-1) at day 28 and 1,002 mu mol electron g protein(-1) min(-1) at day 105. A bacterial community analysis of the anode biofilm revealed that the dominant phylotype, which was closely related to the known exoelectrogenic bacterium, Geobacter sulfurreducens, showed an increase in abundance from 32% to 70% of the total microbial cells. Fluorescent in situ hybridization observation also showed the increase of Geobacter-like phylotypes from 53% to 72%. These results suggest that the improvement of microbial current generation in microbial fuel cells is a function of both microbial cell growth on the electrode and changes in the bacterial community highly dominated by a known exoelectrogenic bacterium during the enrichment process.