Cooling water systems are susceptible to micro-biofouling, more now than ever before. Operating conditions increasingly utilize alkaline pH, less toxic corrosion inhibitors, higher concentration ratios, and use of reclaimed makeup water. Traced biocides are a major step forward in biocontrol technology. Benefits in clued precise documentation and on-line control of product feed and discharge, measurement of actual system consumption, measurement of biocide concentration gradients, and flexibility in the treatment protocol. Results from extensive laboratory studies, field trials and a case history demonstrate the new technology in situations of constant system consumption and upset conditions where biocide is inadvertently depleted.