The case of an extrauterine heterologous malignant mixed müllerian tumor (MMMT) of primary peritoneal origin occurring in a 63 yr old woman is presented. The tumor was a 19 cm, soft, friable mass arising from the serosa of the sigmoid colon and spreading to adjacent pelvic peritoneum. The uterus, tubes and ovaries were uninvolved. It was composed of sarcomatous areas showing cartilaginous and rhabdomyoblastic differentiation and sharply demarcated carcinomatous areas showing endometrioid and serous differentiation. This is the thirteenth reported case of an extragenital MMMT. It demonstrates the pluripotentiality of female pelvic peritoneum to differentiate into tumors resembling those of the genital tract.