Cryptosporidiosis is a nationally notifiable gastrointestinal illness caused by chlorine-resistant protozoa of the genus Cryptosporidium.1 Fecal-oral transmission of Cryptosporidium occurs via ingestion of contaminated recreational water, drinking water, or food, or via contact with infected persons or animals (e.g., cattle). Incidence peaks in late summer and coincides with the summer swimming season.2- 3 The number of nonoutbreak cryptosporidiosis cases reported nationally increased from 3,411 cases in 20042 to nearly 8,300 in 2007 (CDC, unpublished data, 2008). This substantial increase (143%) mirrors the increase in the number of nationally reported cryptosporidiosis outbreaks associated with treated recreational water* venues (e.g., pools, water parks, and interactive fountains): seven reported treated recreational water–associated outbreaks in 2004,4 19 in 2006,5 and, as of September 5, 2008, provisional reports of 26 in 2007 (CDC, unpublished data, 2008). This report describes a communitywide cryptosporidiosis outbreak in Utah that likely was associated initially with treated recreational water venues and subsequently with person-to-person transmission. Cryptosporidium's ability to cause communitywide outbreaks, which is attributable to factors such as its chlorine resistance, underscores the need for more rapid implementation of control measures once an increase in case reporting is noted rather than waiting for an outbreak investigation to implicate a specific source of transmission. Such a response should include (1) pre-outbreak planning and preparation, (2) pre-outbreak adoption of a disease action threshold (e.g., a twofold to threefold increase in cases over baseline), and (3) rapid mobilization of community partners to implement control measures once the threshold is exceeded.