Modifications of the existing methods for the isolation of Giardia spp. cysts from water enabled up to 33% of seeded cysts to be recovered. Using this method, 263 environmental samples were analysed for the presence of cysts, of which 34.4% were positive. All sewage effluents, 85% of recreational waters, 46% of raw potable water samples and 22% of treated water samples contained cysts (range 0.14 - 359/litre). A higher percentage of positive samples occurred in the summer and autumn in raw water, whereas the levels of cysts in treated waters showed little seasonal variation. A fluorogenic vital dye assay, which compares favourably with in vitro excystation, has been developed, and using this assay, viable cysts wire detected in 33% of the positive raw water samples and 42% of the positive treated water samples.