Bioelectrochemical systems (BES) as microbial fuel cells take advantages of microorganisms to convert the chemical energy of organic waste including wastewaters and lignocellulosic biomass into electricity or hydrogen/chemical products. Recently, the discovering that BES can also be harness for product synthesis via microbial electrosynthesis (MES) has greatly expanded the horizon for these systems. MES is a promising strategy which consists in the direct powering of microbial activity with electricity to catalyze conversion of carbon dioxide to transportation fuels and other organic commodities. However, the MES concept is only few years old and scientific data about the functioning and the structure of microbial communities involved are required. In this context, we focused here on the study of communities developed in a BES system. For that, we constructed a dedicated system composed of dual-chamber reactor separated by an anion-exchange membrane and inoculated with biological sludges. Cathode potential was poised at -0.65 volts versus saturated calomel electrode (SCE). Chronoamperometry and cyclic voltammetry experiments were carried out with a multi-channel potentiostat in order to monitor electric activity of the microbial communities. Dynamics of population were achieved by ARISA during experiment and community diversity was investigated at specific time point of interest by sequencing 16S rDNA in bulk phases and in biofilm on the electrodes. Biofilm structures were also visualized using confocal microscopy. Chemical analyses were performed to highlight the metabolic pathways involved. Results showed that current reached intensity of 100 mA/ m². ARISA diversity profiles evolved during the experiment duration showing an adaptation of microbial communities. Pyrosequencing of the 16S rDNA genes revealed specific diversity profiles on the electrodes compared to the bulk. In particular, we observed that Dechloromonas sp. constitute de dominant species (~28% of the total diversity) at the anode whereas this species did not reach more than 1.5% in the bulk and at the cathode. The link between current production, chemical molecules and microbial species diversity is thus discussed. In conclusion, this contribution provides data on the structure and composition of electroactive communities which are crucial for the understanding, the development and the optimization of BES systems.