Recent studies have shown that Cryptosporidium parvum oocysts demonstrate highsusceptibility to low dosages of medium-pressure ultraviolet (UV) light. These investigations have raised several questions, which include determination of minimum medium-pressure UV dosages necessary to inactivate C. parvum oocysts, elucidation of differences (if any) between medium- and low-pressure UV light for inactivating C. parvum oocysts, and evaluation of medium-pressure UV effectiveness in inactivating oocysts suspended in poorer quality water. To compare low- and medium-pressure UV, the authors exposed oocysts suspended in deionized water to UV delivered by either medium- or low-pressure UV lamps at bench scale using a collimated beam apparatus. The applied UV dosages ranged from 3 to 33 mJ/cm2, and oocyst inactivation was assessed using the neonatal mouse infectivity assay. At 3 mJ/cm2, medium-pressure UV showed a 3.4-log inactivation of oocysts, and low-pressure UV showed a 3.0-log inactivation, demonstrating medium- and low-pressure UV did not differ significantly in inactivating C. parvum oocysts. Includes 9 references, tables, figures.