Aims : The research was initiated to confirm earlier ultraviolet (u.v.) light inactivation studies performed on Giardia cysts using excystation as the viability indicator. Following this, a comparison of in vitro excystation and animal infectivity was performed for assessing cyst viability after exposure to low-pressure u.v. irradiation. Methods and Results : Cysts of Giardia muris were inactivated using a low-pressure u.v. light source. Giardia muris was employed as a surrogate for the human pathogen Giardia lamblia . Cyst viability was determined by both in vitro excystation and animal infectivity. Cyst doses were counted using a flow cytometer for the animal infectivity experiments. Using in vitro excystation as the viability indicator, fluences as high as approximately 200 mJ cm(-2) did not prevent some cysts from excysting, thus verifying earlier work. Using animal infectivity, u.v. fluences of 1.4, 1.9 and 2.3 mJ cm(-2) yielded log(10) reductions ranging from 0.3 to greater than or equal to 4.4. Conclusions : Results indicate that in vitro excystation is not a reliable indicator of G . muris cyst viability after u.v. disinfection. Very low doses of u.v. light rendered G. muris cysts non-infective in the mouse model employed. Significance and Impact of the Study : Data presented represent the only complete u.v. inactivation curve for G. muris. This research provides evidence that u.v. can be an effective barrier against Giardia spp. in the treatment of drinking water supplies.