Corrosion of heat exchanger tubes and carbon steel piping at a chemical processing plant had been attributed to a high level of microbiological activity in the cooling water. Two electrochemical biofilm activity sensors with integrated data acquisition and data analysis capabilities were installed in the plant’s cooling system to augment the coupon-based corrosion monitoring activity. Those sensors provided the plant with an on- line measurement and early detection of biofilm activity on metallic surfaces. Sensor response was correla ted with coupon examinations, determinations of biocide residuals, and determination of the numbers and types of microorganisms. Results from the plant monitoring activity are described. These results emphasize the necessity to integrate the various types of field and laboratory data to monitor and effectively control microbiologically influenced corrosion (MIC).