Contaminated tap water led to an outbreak of giardiasis in a New Hampshire community. The epidemiologic studies suggested that the more tap water that was drunk the greater the risk of illness. The community was supplied by two wells. One was found to be contaminated with fecal coliforms. An adjacent pond had beaver activity. Tests of the pond water showed the presence of Giardia intestinalis of the same assemblage, but a different genotype as identified among case-patients. The contaminated well was closed.