The susceptibility of a strain of Legionella pneumophila to disinfection by an organic halamine, free chlorine, and a mixture of the organic halamine and free chlorine was assessed. The organic halamine was found to have superior stability in solution and to exhibit adequate disinfectant potential over a period of 1 month of repeated reinoculations of fresh bacteria. The combined halamine exhibited great potential for use in maintaining closed-cycle cooling water systems free of L. pneumophila.