The inactivation of Cryptosporidium parvum in finished drinking water by medium-pressure UV light (200-300 nm) has been investigated at both the bench scale, using a collimated beam apparatus, and at the demonstration scale, using a Calgon Carbon Corporation Sentinel™ system at the Mannheim Water Treatment Plant, Kitchener, ON, Canada. The viability of the oocysts was assessed using both in vitro (fluorogenic vital dyes (DAPI/PI) and maximized in vitro excystation) and in vivo (neonatal mouse infectivity) assays. In the bench-scale studies, a high degree of inactivation (>4 logs) was found at UV doses as low as 41 mJ cm-2, as assayed by neonatal mouse infectivity; whereas the in vitro surrogate assays showed little or no inactivation at this and higher doses. This indicates that the in vitro assays are unreliable and grossly overestimate the UV doses required to prevent infection by the oocysts in susceptible hosts. The demonstration studies, which were carried out under the NSF/EPA ETV Program, provided results that agreed very well with the bench-scale results and furthermore showed that a UV dose as low as 19 mJ cm-2 provided 3.9 logs inactivation of the Cryptosporidium oocysts.