A new biocidal polymer is described. The compound, poly(1,3-dichloro-5-methyl-5-(4’-vinylphenyl)hydantoin), is prepared in a three-step synthesis from commercial polystyrene. It is insoluble in water and common organic solvents and acts as a contact biocide for contaminated water or air flowed through it. It does not leach organic impurities or levels of free chlorine greater than 1 part per million into flowing water and, thus, may represent a innovation in disinfection technology.