Cooling tower models were used to simulate industrial conditions and evaluate the efficacy of commercially available biocides against adherent (sessile) populations of microorganisms. The cooling tower parameters that were controlled included temperature, pH, cycles of concentration, blowdown rate, recirculation rate, and lighting. The treatment programs were evaluated on the basis of viable cell counts, biochemical monitors, scanning electron microscopy and visual observations. Evaluations of biocide treatment programs are made by monitoring the microbial cells (algal, bacterial, and fungal) in the biofilms that develop on cooling tower slats. The treatments that were successful in maintaining biological control of biofilms in model cooling towers corresponded to commonly recommended programs used in industrial systems. Thus, the model cooling towers offer a relatively simple and meaningful method for evaluating cooling tower biocides in a realistic lab test.