Eighteen dominant isolates from water-cooling systems were exposed to 50mg/l of commercially available bactericides, and the kill percentage was determined after 6 h. Application costs of all bactericides giving an average kill percentage of over 90%, were compared. Low cost bactericides were re-evaluated at cost-equivalent concentrations. Dichlorophen, sulphone, a thiocarbamate and biphenol performed best, killing the full spectrum of isolates cost-effectively. Certain expensive products performed rather poorly, e.g. isothiazoline and MBT. This study highlights the selective action of many bactericides and the inherent resistance of bacteria to a number of different bactericides. This implies the importance of matching bactericides to the dominant bacteria in systems.