The Biox (ENEL patented) on-line monitoring system has been operating at the Vado Ligure power station for two years, in order to control and improve the chlorination treatments of seawater-cooled steam condensers. Some results are shown and discussed in this paper. The monitoring confirmed that Biox is a sensitive detector of the early stage of the biofilm growth, when the loss of the heat exchange in the condenser tubes is not yet detectable. The data also confirmed that Biox system is sensitive to chlorine concentration in the range of standard treatments, (from 0.2 to 0.8 mg/l as Total Residual Oxidant.). Biox monitoring showed that the intermittent chlorination treatment adopted at Vado Ligure power plant was generally effective to keep condenser tubes clean from biofouling. Furthermore, during some periods in winter time, the treatments could be stopped or reduced, without significant biofilm growth, while, during some others periods, even a short lack in the treatments (as short as one or a few days interruption) caused a rapid biofilm growth. In such cases, more intensive chlorination was needed to restore the cleaning condition.