Ozone inactivation ofGiardia muriscysts was investigated in laboratory phosphate buffer (pH 6.7; 22°C) at bench-scale using four ozone dose levels and two contact times. The C3H/HeN mouse model was used to assess the infectivity of ozone treated cysts. This model was found to be a sensitive measure of cyst inactivation to 99.99% inactivation and beyond. It was also observed that ozone rapidly inactivatedG. murisand that ozone concentration appeared to be the most important parameter for predicting ozone inactivation ofG. muriscysts.