It has been demonstrated that the biocidal polymers poly-1,3-dichloro-5-methyl-5-(4'-vinylphenyl)hydantoin and the alkali metal salt and protonated forms of its monochlorinated derivative can be prepared as insoluble porous beads by careful control of the pH in the chlorination step. While the dichloro derivative will be primarily useful in flowing water disinfection applications, particularly for potable water and recreational water, the monochloro compounds will find use in. applications in which disinfection contact time is of lesser importance, such as for recirculating water treatment and odor control. Biocidal efficacy and chlorine stability data have been presented, and the potential use patterns of the several derivatives have been discussed.