||Treatment and disposal of excess sludge from activated sludge processes are becoming more and more expensive. In addition, the existing ultimate disposal of treated sludge, including both incineration and landfill, are facing environmental challenges in densely populated urban cities such as Hong Kong. Therefore, an ideal solution to such a problem is to minimize the excess sludge production by reducing the sludge growth within the processes. This study proposes a novel approach to accomplish the minimization by adding a chemical compound, named 3, 3', 4', 5-tetrachlorosalicylanilide (TCS), to induce microbial futile activity so as to limit sludge growth. The purpose of this study is to examine the feasibility of using such a sludge growth reducing agent in activated sludge cultures. To accomplish this goal, the following works were carried out: 1) to investigate the effect of TCS on sludge growth in the batch activated sludge cultures that were cultivated without TCS addition; 2) to examine the response of the batch culture to the introduction of TCS at different doses during a 30-day operation period, in light of microbial respiratory activity and sludge growth; and 3) to further evaluate the effectiveness of TCS on the reduction of excess sludge in a continuous-flow culture of activated sludge. The results show that in the batch tests with 0.8-ppm TCS, a 40% reduction of the sludge growth was achieved for the batch activated sludge culture previously cultivated without addition of TCS. In the batch cultivation of activated sludge cultures with 1.0-ppm TCS addition, the excess sludge production rate was also reduced by approximately 40%. The substrate removal capability of the culture was not affected adversely during the 30-day cultivation period. These findings have been reconfirmed by the continuous cultivation of activated sludge culture with TCS addition. The cause of the excess sludge reduction induced by TCS has been found to be associated with the enhancement of microbial respiring activities and an increase of active cell portion over the total microbial population in the sludge, investigated by the SOUR measurement and microbial DAN staining techniques such as 4',6-diamidino-2-phenyl indole (DAPI) and 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) staining methods. When both the batch and the continuous cultures were cultivated with 1.0-ppm TCS addition, their microbial respiring activities were increased by 30-42%, while the active cell populations were increased from 6% to 10%. It was also proved that TCS does not undermine the treatment performance of both the batch and the continuous cultures in term of effluent quality. Based on the above findings, it can be concluded that it is feasible to apply TCS to achieve a reduction of excess sludge in activated sludge cultures.