The invention provides a biocidal composition for controlling the growth of microorganisms in swimming pools and other aqueous systems. The composition includes: an antimicrobial substance for controlling microbial growth； an ionene polymer； and a synergist. The antimicrobial substance may be selected from the group consisting of: quaternary ammonium compounds such as cetylpyridinium chloride and benzalkonium chloride； isothiazolinone-derived compounds such as methylisothiazolinone； methylene bisthiocyanate； dibromonitrilopropionamide； and 2-bromo-2-nitropropane-1, 3-diol. The ionene polymer is preferably: poly[(oxyethylene(dimethyliminio)-ethylene(dimethyliminio)-ethylene)dichloride]. The synergist may be selected from the group consisting of triethanolamine, diethanolamine, ethanolamine, ethanol, and triethylamine. Preferably the synergist is triethanolamine.
