We report a case of malignant mixed müllerian tumor (MMMT) (carcinosarcoma) of the right fallopian tube in a 69-year-old woman presenting with abdominal pain and an adnexal mass. The patient underwent total abdominal hysterectomy, bilateral salpingo-oophorectomy, omentectomy, received adjuvant chemotherapy and is without evidence of disease 12 months postoperatively. The tumor involved the fallopian tube and was composed of in situ and invasive high-grade serous and undifferentiated carcinoma, leiomyosarcoma, rhabdomyosarcoma and undifferentiated sarcoma. Immunohistochemically, the epithelial and mesenchymal cells expressed CD56, Leu-7 and p53. The epithelial elements expressed nuclear WT1 and calretinin while the mesenchymal cells showed negative nuclear and strong cytoplasmic staining. HBME was observed focally in carcinoma. The expression of mesothelial-associated antigens WT1, calretinin and HBME in MMMT likely reflects the common embryologic derivation of the mesothelium and urogenital ridge. Loss of nuclear WT1 expression in the mesenchymal component may be involved in MMMT tumorigenesis.