The dynamic behavior of a mediator-less, microbial fuel cell (MFC) was studied as a continuous biochemical oxygen demand (BOD) sensor. The response time and the sensitivity were analyzed through the step-change testing of the fuel concentration. The MFC of 25 ml had the shortest response time of 36+/-2 min at the fuel-feeding rate of 0.53 ml min(-1) and the resistance of 10 Omega A smaller MFC of 5 ml had a response time of 5+/-1 min.