Genetically-engineered microbial biosensors have yet to realize commercial success in environmental applications, due in part to difficulties associated with transducing and transmitting traditional bioluminescent information. The use of bioelectrochemical systems (BESs) in biosensing applications allows for a direct electronic output that can be more easily incorporated into devices for remote environmental monitoring. Herein, we describe the first BES-based biosensor with genetically encoded specificity for a toxic metal. By placing an essential element of the metal reduction (Mtr) pathway of Shewanella oneidensis MR-1 under the control of an arsenic-sensitive promoter, we have engineered a strain that produces increased current in response to arsenic when inoculated into an BES. When operated as a chemostat with a hydraulic retention time of 7 h, our BES-based biosensor has a response time of 24 h and a lower detection limit on the order of 100 μM arsenite. To heighten the sensor's sensitivity to arsenic, we have begun characterizing an alternative sensing strain with improved translational efficiency. This ability to tune analyte sensitivity—along with the reliability of our continuous assay and the simplicity of the transcriptional circuit required for BES-based biosensing—suggests that similar sensing systems may be readily developed for both environmental deployment and on-line process control.