This part of ISO 10705 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 preconcentration step may be necessary for which a separate International Standard has been developed. The method is also applicable to shellfish extracts. NOTE It is desirable for International Standards to be adopted as widely as possible. This part of ISO 10705 includes reference to alternative procedures which obviate the need for expensive materials or equipment which may not be readily available in developing countries. Use of these alternatives will not affect the performance of this method.