Monitoring and controlling microbial growth is vital to the success of a comprehensive cooling water treatment program. Hydrocarbon ingress to the cooling water requires novel approaches to control microbiological fouling. An understanding of the factors that affect microbial growth rates and the components of an effective bacterial mitigation program can minimize the severity of biofouling in the cooling water system. Experience obtained over the years has allowed the development of guidelines that minimize microbiological growth and prevent shut down of the plant during hydrocarbon excursions. This paper examines one case study that illustrates the performance of a biocide treatment program and the importance of correct monitoring in a hydrocarbon environment.