A rare case of malignant mesenchymoma in a 58-year-old woman is described. The tumor grew rapidly for a month and was located in the mesosigmoid. The resected tumor measured 25 X 20 X 20 cm and weighed 5,000 g. It metastasized into the great omentum within four months after operation. Histologically, this tumor was composed of undifferentiated sarcomatous elements, but in some parts revealed patterns of differentiation, including leiomyosarcoma, chondrosarcoma and osteosarcoma. In areas of osteosarcoma, the bone tissue varied from immature to mature. The histology of the recurrent tumors was similar to that of the first tumor. We considered that undifferentiated sarcomatous elements contained pluripotential cells which could differentiate multiple, mature malignant mesenchymes.