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This study investigated the effect of external resistance (R_ext) on the dynamic evolution of microbial communities in anodic biofilms of single-chamber microbial fuel cells fueled with acetate and inoculated with municipal wastewater. Anodic biofilms developed under different R_ext (0, 330 and 1000 ohms, and open circuit condition) were characterized as a function of time during two weeks of growth using 16S rRNA gene sequencing, cyclic voltammetry (CV) and fluorescence microscopy. The results showed a drastic difference in power output of MFCs operated with an open circuit and those operated with R_ext from 0 to 1000 ohms. Two steps during the bacterial community development of the anodic biofilms were identified. During the first four days, nonspecific electroactive bacteria (non-specific EAB), dominated by <i>Pseudomonas</i>, <i>Acinetobacter</i>, and <i>Comamonas</i>, grew fast whatever the value of R_ext. During the second step, specific EAB, dominated by <i>Geobacter</i> and <i>Desulfuromonas</i>, took over and increased over time, except in open circuit MFCs. The relative abundance of specific EAB decreased with increasing R_ext. In addition, the richness and diversity of the microbial community in the anodic biofilms decreased with decreasing R_ext. These results help one to understand the bacterial competition during biofilm formation and suggest that an inhibition of the attachment of non-specific electroactive bacteria to the anode surface during the first step of biofilm formation should improve electricity production.