The cysts of the nonpathogenic soil and water amoeba, Naegleriagruberi strain (NEG), were inactivated using aqueous ozone in batch reactors at controlled temperatures (5–30°C) at pH 7. The semi-log plots of survival ratio vs. time consisted of an initial shoulder followed by a linear portion. The log-log plots of ozone concentration against time for 99 percent inactivation were essentially linear. The relationship between the overall inactivation rate and absolute temperature was well expressed by the Arrhenius theory. An empirical relationship was developed between the extent of cyst inactivation, ozone dose, temperature, and contact time.