Microbial fuel cell (MFC) technology has enabled new insights into the mechanisms of electron transfer from dissimilatory metal reducing bacteria to a solid phase electron acceptor. Using solid electrodes as electron acceptors enables quantitative real-time measurements of electron transfer rates to these surfaces. We describe here an optically accessible, dual anode, continuous flow MFC that enables real-time microscopic imaging of anode populations as they develop from single attached cells to a mature biofilms. We used this system to characterize how differences in external resistance affect cellular electron transfer rates on a per cell basis and overall biofilm development in Shewanella oneidensis strain MR-1. When a low external resistance (100 Omega) was used, estimates of current per cell reached a maximum of 204 fA/cell (1.3 x 10(6) e(-) cell(-1) sec(-1)), while when a higher (1 MOmega) resistance was used, only 75 fA/cell (0.4 x 10(6) e(-) cell(-1) sec(-1)) was produced. The 1 MOmega anode biomass consistently developed into a mature thick biofilm with tower morphology (>50 microm thick), whereas only a thin biofilm (<5 microm thick) was observed on the 100 Omega anode. These data suggest a link between the ability of a surface to accept electrons and biofilm structure development.