An axenic culture of Giardia was established from a sample of infected intestine obtained following autopsy of a sulphur-crested cockatoo (Cacatua galerita). The cockatoo recently captured in the wild and with good muscle tone died along with several other cage mates, apparently of an overwhelming, acute infection of Giardia. Trophozoites which established in the traditional, axenic Giardia medium (TYI-S-33 with supplementary bile) were morphologically identical to G. duodenalis. When outbred Quackenbush Swiss neonatal mice were infected with trophozoites a chronic infection was established and parasites were still present at 38 days post-inoculation. Weight gain by infected mice was reduced by 20%, thus mimicking failure-to-thrive syndrome in children, and maximum parasite load was more than 3-fold higher in comparison with other G. duodenalis strains. Analysis of the electrophoretic karyotype, rDNA and hybridization studies together with Giemsa- and trichrome-stained samples, and scanning electron microscopy indicated that the bird-derived Giardia belonged to the duodenalis group. This is the first report of infection of mammals with Giardia isolated from a bird. These data may have potentially serious implications for contamination of watersheds and establishment of zoonotic infections.