A large communitywide outbreak of Cryptosporidiosis occurred in Utah. Approximately 80% of the cases had used recreational waters within 14 days prior to their illness onset. As the outbreak evolved, beginning in mid August, and increasing percentage of the cases reported no recreational water exposure, but did report contact with persons who had diarrhea. This suggested that more of the later cases were due to person-to-person transmission. The outbreak disproportionately affected children less than age five.