In order to increase power generation, two individual microbial fuel cells were stacked together either in series or in parallel. The maximum power densities obtained from MFC 1 and MFC 2 were 464 and 542 mW/m(2) (normalized to the surface area of the anodic electrode, which was 30.25 cm(2)), corresponding to current densities of 876 and 1,338 mA/m(2), respectively. The microbial fuel cells stacked in series produced an open circuit voltage of 1.3 V and a working voltage of 1.22 V (1,000 Omega). The maximum power of 3.1 and 2.9 mW were obtained in serial and parallel connections, corresponding to current values of 3.9 and 6.26 mA, respectively. However, voltage reversal was encountered when the stacked microbial fuel cells were operated in batch mode for 150 h in series.