Giardia intestinaliscysts were detected in faeces from 12.9% of 124 brushtail possums (Trichosurus vulpecula) trapped in Auckland, Hawke's Bay, the Wairarapa and Wellington; in faecal material from 61.0% of 77 ship rats (Rattus rattus) from areas of Auckland, Hawke's Bay and Wellington; and in faecal material from 25.3% of 182 house mice (Mus musculus) from the North Island. Possums and rodents in forested areas of Auckland and Wellington city water catchments were infected. The role of wild animals in maintaining and spreadingGiardiain New Zealand is discussed.