An unusual case of a müllerian adenosarcoma of the peritoneal mesothelium in a 32-year-old pregnant woman is described. On emergency laparotomy, a tumor was found in the pelvis connected with the right broad ligament only, and was removed. Light and electron microscopic examination showed a rare example of an extrauterine müllerian adenosarcoma composed of a stromal sarcoma, muscle tissue resembling primitive myoblasts, and sex-cord-like tubular lumens. The patient was followed, and after 22 months the primary surgery was reexplored. On laparotomy, no evidence of disease was found, and the ovaries were tumor-free. Histologic examination of the pelvic peritoneum demonstrated small areas of adenosarcoma resembling elements of the previous tumor. Müllerian adenosarcoma is a rare tumor of müllerian origin, and its less aggressive behavior as compared with mixed müllerian sarcoma is warranted. The better prognostic outlook of müllerian adenosarcoma is probably related to the histologic structure, but depends in addition on the size of the tumor and the site of origin. No case of peritoneal origin of müllerian adenosarcoma has been previously reported; we discuss this unusual location.