Mixed mesodermal tumor of the ovary is very rare, only 10 cases having been reported to date in Japan. We recentry experienced a case of this type of tumor and studied it cytologically and electron microscopically. The patient was a 61-year-old housewife (gravida 2, para 2) who had experienced menopause at the age of 51 years. She received medical examination because of a lower abdominal mass. Ultrasonography and CT revealed a large intrapelvic tumor and ascites. Since the CA 125 level was 704 U/ml, a malignant ovarian tumor was suspected, and bilateral adnexectomy, total hysterectomy, resection of the intestinal area adhering to the tumor plus end-to-end anastomosis, and omentectomy were performed. After 5 courses of chemotherapy, the CA125 level was restored to normal. The patient is now being followed up on an outpatient basis. The resected specimen, the size of a small newborn head, was a soft tumor with a necrotic appearance which adhered to and infiltrated the sigmoid colon. There was about 600 ml of hemorrhagic ascites. Touch smear cytology demonstrated isolated sarcoma cells and endometrioid adenocarcinoma cells showing an accumulated ductal arrangement. Histologically, the tumor was considered to be carcinosarcoma showing a mixture of endometrioid adenocarcinoma and sarcoma. Since transmission electron microscopy of the sarcoma element disclosed thick and thin intracytoplasmic filaments in some tumor cells, the cytoplasm, those were diagnosed as rhabdomyosarcoma cells. By scanning and transmission electron microscopy of adenocarcinoma cells on the same touch smears, grape-like aggregates of tumor cells, bearing dense or sparse microvilli, were evident. Also, there were nucleoli in the marginal areas of large nuclei, intercellular junction complex, and unclear organelles. The histogenesis of this tumor was discussed on the basis of the above findings and the related literature.