Cooling towers are potential sources for the amplification and dissemination of legionellae. Public health risks posed by legionellae may be eliminated via preventive methods. Biocide application in cooling towers is the most common and critical prevention strategy against legionellae. Successful results may be achieved by pre-determining microbial inhibitory concentration and minimum contact time for a given biocide. In the current study, efficacy of various biocides was evaluated in terms of dosage and contact time, against various Legionella pneumophila strains, in vitro. Results indicated that quaternary ammonium compounds were effective at lower dosages than those recommended by the manufacturer. K4 kloran, an oxidizing agent, was the least effective biocide. It was found that the efficacy of biocides was directly related to applied dosages and the origin of the target strains (standard or environmental).