Cryptosporidium is a parasitic protozoan found in water sources and spread through the fecal-oral route. Cryptosporidiosis is characterized by gastroenteritis and is increasingly associated with recreational water sources. On December 3, 2010, Niagara Region Public Health was informed of a laboratory-confirmed case of Cryptosporidium. Over the subsequent two weeks, a total of three additional laboratory-confirmed cases were reported. All cases had visited the same water park in Niagara Region, Canada, over November 14-16, 2010. A total of 12 cases associated with the outbreak ranged in age from 1 to 66 years. This article describes the outbreak, environmental investigation, and control measures. The environmental investigation revealed that the ultraviolet disinfection system was offline on November 14, 2010, which may have allowed for the transmission of Cryptosporidium to bathers. Further research into the detection of Cryptosporidium outbreaks and regulations and guidelines for water park operators may help to decrease future outbreaks.