Hydatid cyst is an important endemic zoonosis in Iran. It may be seen in various organs of body. Musculoskeletal system is rarely involved by hydatid cyst, the larval form of Echinococcus granulosus. On clinical basis, it may resemble any soft tissue tumor. A 70-years old housewife living in Ardoghesh, a village in Neyshabur, northeast Iran, was admitted to general surgery clinic because of a painless mass in the back of her left thigh. This case emphasizes that hydatidosis should be included in differential diagnosis of any soft tissue mass especially in regions where hydatidosis is endemic.