The potential of 3,3',4',5-tetrachlorosalicylanilide (TCS) addition to an activated sludge continuous process to reduce excess sludge production by disrupting coupling between anabolism and catabolism was investigated. TCS was chosen as a metabolic uncoupler for continuous test in a lab-scale completely mixed activated sludge process. TCS reduced sludge yield by approximately 30% at a dosage of 40 mg/day. Substrate removal capability was not adversely affected by the presence of TCS, but effluent nitrogen concentration increased during the 60-day continuous operation. Sludge settleability of treated and control samples was qualitatively comparable and not significantly different. Microbial activities in terms of specific oxygen uptake rate were also enhanced, and the microbial population was altered. The results suggest that TCS is an effective chemical uncoupler that reduces sludge yield; process performance was not significantly affected by introduction of the uncoupler.