A communitywide outbreak of gastrointestinal illness due to Giardia lamblia infection occurred in the city of Berlin, New Hampshire, during April and May 1977. The clinical, epidemiologic, and laboratory aspects of this outbreak are described here. In 213 predominantly symptomatic cases of G. lamblia infection diagnosed at a local hospital laboratory in a 6-week period, illness was characterized by prolonged diarrhea (median duration 10 days) and 13% of symptomatic infections required hospitalization. Treatment with either quinacrine or metronidazole was generally followed by symptomatic improvement. A communitywide survey of the city residents revealed that the majority (76%) of G. lamblia infections occurring during the epidemic period were asymptomatic and ran a self-limited course without treatment. No significant secondary, person-to-person spread occurred and no enteric pathogens other than G. lamblia were implicated. Water was epidemiologically implicated as the most likely source of infection with Giardia cysts being demonstrated in samples of treated water as well as raw source water. Evidence supported the occurrence of two simultaneous outbreaks in this city which is supplied by two largely independent water supply systems. Inspection of the two water treatment facilities revealed several defects which permitted untreated (raw) water to mix with treated water. Human or beaver could have been responsible for contaminating source water with Giardia in this outbreak. A marked reduction in both clinical and subclinical giardiasis was apparent two months after onset of the outbreak, apparently as a result of measures applied to interrupt waterborne transmission of Giardia.