Abstract: A composition comprising a water-dispersible ionene polymer and a water-dispersible di-lower-alkyl ammonium halide polymer (for example, a polymer of dimethyl diallyl ammonium chloride) is disclosed. These polymers are present in the composition in a ratio effective to function synergistically as an algistatic or algicidal agent when an effective amount of the composition is dispersed in a body of water. Examples of suitable bodies of water are swimming pools, spas, lakes, rivers, or water in an industrial water system, such as a cooling water system. Also, a method of treating water which is susceptible to the growth of algae is disclosed. The method is carried out by incorporating in the water an at least algistatic amount of the composition.