Water samples from household tanks in several districts of Alexandria, were examined for protozoa contamination and study to the effect of ozone and chlorine on their viability. The parasitic protozoa encountered were Giardia (56%), Cryptosporidia (50%), Blastocystis (12%), Cyclospora (9%) and Microsporidia (3%). Distilled water was treated by ozone or chlorine. Ozonated-water samples were prepared at a concentration of 1 ppm after contact times with ozone of three, five, seven and nine minutes. Chlorine concentrations were 4 and 8 ppm. Parasites were incubated overnight at room temperature with the treated distilled water. Viability of the parasites was assessed by examining intestinal sections of infected animals. The best results were obtained by using ozonated water after a contact time of nine minutes. Giardia, Cryptosporidia and Microsporidia were completely inactivated, while infectivity of Cyclospora and Blastocystis was markedly reduced. It is suggested that ozone at an appropriate concentration inactivate pathogenic protozoa in water.