Aims: To determine the effect of solar radiation on Cryptosporidium parvum in tap and environmental waters. Methods and Results: Outdoor tank experiments and a cell culture infectivity assay were used to measure solar inactivation of C. parvum oocysts in different waters. Experiments conducted on days with different levels of solar insolation identified rapid inactivation of oocysts in tap water (up to 90% inactivation within the first hour). Increased dissolved organic carbon content in environmental waters decreased solar inactivation. The role of solar ultraviolet (UV) in inactivation was confirmed by long-pass filter experiments, where UV-B was identified as the most germicidal wavelength. Reductions in oocyst infectivity following solar radiation were not related to a loss of excystation capacity. Conclusions: Solar UV can rapidly inactivate C. parvum in environmental waters. Significance and Impact of the Study: This is the first study to assess natural sunlight inactivation of C. parvum oocysts in surface waters and drinking water using an infectivity measure and determines the wavelengths of light responsible for the inactivation. The findings presented here provide valuable information for determining the relative risks associated with Cryptosporidium oocysts in aquatic environments and identify solar radiation as a critical process affecting the oocyst survival in the environment.