A method has been developed combining immunofluorescence and phase-contrast to microscopically locate and identify Giardia cysts among particulates filtered from water samples. Selective staining of cysts with anti-Giardia antiserum and a fluorochrom enhances the contrast between cysts and other sample constituents and allows quick location of cysts by their green fluorecence. Their identity can then be confirmed with phase-contrast observation of size, shape, and internal morphology. The method was successfully tested during recent outbreaks in Pennsylvania and Massachusetts. However, when chlorinated water samples were tested, confirmation of presumptive Giardia cysts was difficult or impossible.