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 the 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.