Biofouling control in coastal power stationsis generally achived through the use of chlorination. However, chlorine reacts with natural organic matter, leading to the formation of several chlorinated by-products such as trihalomethanes (THMs). Environmental dischare of THMs is of concern, as these compounds have been reported to carcinogens and mutagens. Usgae of chlorine dioxide as an alternative biocide is an effective way of controlleing biofouling. This paper compares the formation and distribution of THMs in the cooling water discharge of a coastal power plant operating on the east coast of India under conditions of chlorination and chlorine dioxide addition. It is shown that use of chlorine dioxide as an alternative biofouling control agent results in reduced THMs formation and environmental release.