Polymeric phosphonium salts with different side-chain length between the main chain and the active group, poly[4-(2-tributyl-phosphonioethyl)styrene chloride-co-4-(2-chloroethyl)styrene] and poly[4-(3-tributylphosphoniopropyl)styrene chloride-co-4-(3-chloropropyl)styrene], were prepared and their antibacterial activities against Staphylococcus aureus and Escherichia coli were examined by the viable cell counting method in sterile distilled water. The antibacterial activity was found to decrease as the side-chain length increased. The most significant finding is a peculiar concentration dependence of the antibacterial activity, which demonstrated the presence of an optimal concentration. The observed specific effects of the side-chain length and the concentration on the antibacterial activity seem to be ascribed to the state of aggregation of the polycationic biocides in aqueous solution.