The genus Giardia has been subdivided by Filice (1952) into 3 species, G. agilis, G. muris, and G. duodenalis, based on the morphology of the median body and subtle variations in the dimensions of trophozoites. Giardia trophozoites were isolated from the small intestine of budgerigars (parakeets) and examined morphologically with light and scanning electron microscopy. These trophozoites, like other Giardia spp., possessed a flattened dorso-ventral shape, 8 flagella, and an adhesive disc on the ventral surface. The presence of a claw hammer-shaped median body suggested classification of these trophozoites as G. duodenalis. However, unlike any known members of G. duodenalis, the Giardia trophozoites from budgerigars were morphologically distinct in that they lacked the ventrolateral flange and therefore did not have a marginal groove bordering the anterior and lateral border of the adhesive disc. This distinct morphology clearly indicated that trophozoites from budgerigars should be considered as a separate species, G. psittaci. Our evidence has demonstrated that median body shape cannot serve as a sole criterion for speciation of Giardia. In addition, if other avian species of Giardia also resemble G. psittaci, then this would suggest that evolutionary divergence has occurred in the genus Giardia.