This International Standard specifies a method that is applicable for the detection and enumeration of Cryptosporidium oocysts and Giardia cysts in water. It is applicable for the examination of surface and ground waters, treated waters, mineral waters, swimming pool and recreational waters. This method does not allow identification to species level, the host species of origin or the determination of viability or infectivity of any Cryptosporidium oocyst or Giardia cyst which may be present. These procedures are for use by experienced analysts who have successfully completed competency tests prior to commencing analysis. In addition, such analysts should continue to demonstrate competency by examining seeded samples at regular intervals and taking part in external quality assurance schemes. NOTE Bodies resembling Cryptosporidium or Giardia in morphology can be present and these may be mistaken for oocysts or cysts. Results should be interpreted with care. Where there is doubt about the identity of oocysts or cysts or where an unusually high result is obtained, it is advisable to have the slides examined by experts from other laboratories to confirm or refute the findings.