A marked increase in reports of Cryptosporidiosis occurred during the summer in Campbell County, Wyoming. Swimming in community swimming pools and exposure to Keyhole Reservoir were common exposures reported by case patients. Accidental fecal releases at local pools may have contributed to the spread of the illness. Hyperchlorination of community pools appeared to reduce disease transmission, but could not be confirmed. This community-wide outbreak mirrored an outbreak of cryptosporidiosis that occurred in both Campbell and Crook Counties in 2006. Investigators suspected that persons with undiagnosed infections and asymptomatic carriers may have contributed to the reemergence of Cryptosporidium in Campbell County in 2007.