Polysiloxanes with 3-(alkyldimethylammonio)propyl pendant groups were synthesized by quaternization of n-octyldimethylamine or n-dodecyldimethyl amine with linear polysiloxanes containing 3-chloropropyl groups and/or 3-bromopropyl groups attached to silicon atoms. The precursor polysiloxanes, poly[(3-chloropropyl)methylsiloxane] homopolymer and various copolymers containing (3-halogenopropyl)methylsiloxane and dimethylsiloxane units, were obtained by equilibrium cationic polymerization of linear and cyclic siloxanes with (3-halogenopropyl)methylsiloxane units. The polysiloxanes bearing quaternary ammonium salts (QAS) showed bactericidal activity against bacteria such as Escherichia coli and Aeromonas hydrophila when incorporated in a polysiloxane network. The activity was retained after 66 days of immersion in water. The QAS-containing polysiloxanes are also active in aqueous solution. (C) 2000 John Wiley & Sons, Inc.