An animal model for giardiasis was developed using Giardia muris in Swiss albino mice. Intraesophageal inoculation of G. muris cysts caused a reproducible pattern of infection, with trophozoite and cyst counts reaching a maximum on days 5 to 14 after cyst inoculation and thereafter showing a progressive decline. Spontaneous resolution of infection occurred in most mice after 21 to 28 days. When compared to uninfected controls, Giardia-infected mice had significant impairment of weight gain and a significant reduction in the villus to crypt ratio of jejunal mucosa. Although maximal trophozoite and cyst counts were independent of the size of the cyst inoculum, those mice receiving inoculations of larger numbers of cysts showed earlier attainment of maximal counts, greater impariment of weight gain, and earlier and more severe small bowel changes than mice receiving inoculations of smaller numbers of cysts. This model offers unique opportunities for study of this poorly understood gastrointestinal parasite.