Using animal infectivity tests, the authors evaluated a water disinfection device, UV Waterworks (UVW), for its ability to inactivate Cryptosporidium parvum oocysts. The UVW employs low-pressure, germicidal ultraviolet (UV) light, delivering a dose of approximately 120 millijoules per square centimeter (mJ/cm2) under ideal water conditions at a flow rate of 4 gallons per minute (gpm). Dechlorinated tap water containing live oocysts was passed through the UVW at 4 gpm. The oocysts were captured on a filter, separated from the filter, and concentrated into inocula—10 microliters (microL) each, containing between 10(3) and 10(7) oocysts—which were administered orally into 60 neonatal mice. After one week, the mice were killed, and sections of their terminal ilea were analyzed microscopically for signs of Cryptosporidium infection. In spite of the high dose of oocysts, none of the mice showed signs of infection. A process control run with the UV lamp off resulted in 95 percent infection at a dose of 10(3) oocysts per inoculum. The calculated reduction in oocyst infectivity from passage through UV Waterworks was at least 5.4 orders of magnitude. The authors conclude that exposure to low-pressure UV at 120 mJ/cm2 effectively disables Cryptosporidium.