Fourteen calves, 12 days to 12 weeks old, were treated for diarrhea. Fecal examination for parasitologic, bacteriologic, or viral infection revealed Giardia in all calves; rotavirus and coronavirus were found in some calves. Thirteen affected calves were treated orally with dimetridazole (50 mg/kg of body weight, daily, for 5 days), with complete resolution of the diarrhea and elimination of the Giardia. Giardiasis should be considered as an etiologic agent of diarrhea in calves.