Amongst the techniques suggested for the determination of Giardia cyst viability, the use of the fluorogenic dyes, fluorescein diacetate (FDA) and propidium iodide (PI) is the most often recommended, even though it appears to overestimate the number of viable cysts. In the present study, the replacement of FDA with 4′,6-diamidino-2-phenylindole (DAPI) allowed simultaneous direct immunofluorescence with monoclonal antibody labelled with fluorescein isothiocyanate (MAb-FITC). Under these conditions, it was possible both to quantify the cysts according to the immunofluorescence technique, and to appreciate their viability by using fluorogenic dye staining (DAPI and PI) and differential interference contrast (DIC) microscopy. This method proved to be significantly better than the counting methods normally suggested. The technique has been applied to Giardia cysts recovered from faeces and wastewater sludge.