Giardiasis is an important human parasitic disease. Giardia is a genus composed of binuclear flagellate protozoa. Giardia duodenalis is a parasitic species for a wide range of vertebrates, including humans. Heterogeneity in G. duodenalis has been shown by serological, biochemical, and molecular analysis. In the present study, the possible genetic similarity between Giardia in sheep and humansand their probable zoonosis was investigated. Direct examination and formalin ether concentration technique were performed on the contents and tissues of sheep intestines. The gradient sucrose method was applied to collect and purify cysts, and DNA extraction was performed by the phenol-chloroform method. Only very small amounts of DNA could be extracted after repeated freezing, thawing and suspension with lysis buffer, after which polymerase chain reaction (PCR) was performed for DNA amplification by primers that were designed for Giardia of human origin. The gene, "triose phosphate isomerase" (tim or tpi), was selected as the molecular marker and two sets of primers (PM290, PM924) were used. We examined 308 sheep stool samples in our study, including 21 positive samples. Cultures for Giardia were negative. Three sheep isolates were determined by a 290 base pair (bp) amplicon that were similar to certain human types. The similarity of the sheep and human genomic characters of Giardia implies the possibility that there is transmission of these protozoa between humans and sheep.