A large outbreak of Cryptosporidiosis was associated with exposure to a water sprinkler fountain at the Minnesota zoo. Case-patients reported exposure to the water fountain between June 24 and July 11. The initial cases were persons who had attended a birthday party. Case finding identified many more cases. The water sprinkler fountain had initially been built as a decorative display, but people often ran through the water spray. The water in the fountain was chlorinated, however the water filter had not been flushed.