In order to reduce the environmental impact of biocide for the control of biofilm formation in cooling water circuits, "environmentally friendly" biocides have been developed; however, they are generally more expensive than traditional chemicals. It is imperative, therefore, that the minimum quantity of biocide is employed so that costs are kept to a minimum. To achieve this objective, optimum dosing strategies are required. Using a pilot plant in conjunction with a monoculture of Pseudomonas fluorecsens as the biofouling bacterium, tests were carried out using a proprietary biocide to investigate the effects of dose concentration, duration, and frequency of dosing and fluid mechanics on biofilm control. With four fifteen-minute applications per day at a peak concentration of 16.8 mg/l, it was not possible to inhibit biofilm development. Control was effected, however, by doubling the peak concentration using a short dosing period. As would be expected, concentration was shown to be a critical factor for control. A boicide concentration below that for growth inhibition seemed to enhance biofilm formation. Increasing the frequency of dosing is only effective if the concentration employed is biofilm growth-inhibiting.