Bio-fouling refers to the undesirable accumulation of a biotic deposit on a surface. This consists of an organic film composed by micro-organisms embedded in a polymeric matrix of their own making. Complex fouling deposits, like those found in industrial environments, often consist of bio-films in intimate association with inorganic particles, corrosion products and macro-fouling organisms. The article focuses on the possibility to minimize the problems related to micro and macro-fouling in once-through cooling systems and desalination plant by shot or continuous injection of chlorine dioxide into the seawater at the intake. A special on line monitoring instrument, installed after chlorine dioxide injection point, permits to check the fouling growth due to marine micro-organism and, in the meantime, the residual of chlorine dioxide. Chlorine dioxide has proved to be an efficient antifouling agent used 1 h/da maintaining residual Of ClO2 in the range of 0.05 to 0.1 mg/L during the dosing shots or 10-12 h/da without any residual at several power station and petrochemical plant in Mediterranean sea, which use seawater both in condenser and evaporators to produce demineralised water. Beside a description of chlorine dioxide chemistry, the article reports 4 power station dosage programmes (one using also river water cooling tower during the summer) and 1 petrochemical plant application. Three different technologies for on site generation of chlorine dioxide have been reported. A full case history selected among the 4 power station is also reported.