Specifies a method for the detection and enumeration of bacteriophages infecting Bacteroides fragilis by incubating the sample with an appropriate host-strain. The method is applicable to all kinds of water, sediments and sludge extracts, where necessary after dilution. In the case of low phage numbers, a pre-concentration step may be necessary for which a separate International Standard has been developed. The method is also applicable to shellfish extracts.