We report the development of microbial populations and changes in their electrochemical production during a 2-month study of a two-chamber microbial fuel cell (MFC). The original inoculum was taken from anaerobic enrichment cultures with soil as the inoculum, and lactate as the exogenous electron donor. Power density (PD), coulombic production (CP), and coulombic efficiency (CE) increased rapidly, reaching high values (320 mW m(-3), 65 Q, and 12.5%, respectively) in 12-16 days. Under these conditions, several major microbial taxa dominated the anode population. The medium solution in the cathode chamber decreased with aeration, resulting in a decrease in PD to 55 mW m(-3) at day 20. Refilling the cathode chamber around day 30 resulted in restoration of the PD, CP and CE to values equal to or greater than those previously observed. However, after the change in conditions, a marked change in community structure was observed, and high levels of acetate were seen in the anode chamber of the fuel cell for the first time. At day 35, a series of lactate concentrations were used, beginning with low levels and increasing to the 20 mM level originally used (day 46), the PD decreased but was stable at 150 mW m(-3) and the acetate concentration in the anode stabilized at about 35 mM. Under these conditions, new major population structures, which were closely related to Propionibacterium, Clostridium, and uncultured bacteria, were observed in the anode. These results suggested that the flexibility of community structure was important for sustainable electricity production. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.