Five black-tailed prairie dogs showed weakness. emaciation and anemia during the lairage period in May 2005. Among them, one adult male and one female prairie dogs were found dead. The stamp smear for revealed a tremendous number of Giardia trophozoites. Histopathological examinations of the intestine showed slightly inflammatory changes and cystic enlargement of the crypts in the duodenal and jejunal mucous membranes. From these findings, this disease was diagnosed as giardiasis in black-tailed prairie dog.