The sensitivity to chlorination and to UV-irradiation of bacteriophage B40-8, which infects Bacteroides fragilis, was evaluated in comparison to that of Escherichia coli, Streptococcus faecalis, poliovirus 1, simian rotavirus 11 and coliphage f2. The results indicated that viruses persisted longer than bacteria in the presence of both disinfectants. Phage B40-8 was the most resistant microorganism to chlorination while coliphage f2 was the most resistant to UV-irradiation. In the latter, phage B40-8 was nevertheless as resistant as poliovirus and rotavirus. As expected, all microorganisms were more resistant to inactivation in sewage water than in tapwater.