The authors report a rare case of hydatid cyst of the psoas muscle in a 28-year-old woman, operated for vertebrospinal hydatid cyst. The patient presented with a left iliac fossa mass. Ultrasonography and computed tomography contributed to the preoperative diagnosis, despite negative hydatid serology. A retroperitoneal approach by left lumbotomy with partial pericystectomy was performed. In the light of this case, the authors discuss the diagnostic and therapeutic problems raised by hydatid cyst of the psoas muscle.