One case of cystosarcoma phyllodes is reported. Histologically, the neoplasm was composed of two elements: benign epithelial cells and malignant stromal cells. The stromal cells, spindle shaped, grow in solid sheets or in sparcely cellular, myxomatous and alcianophilic areas. Scanning electron microscopy demonstrates a mucous secretion, showing porelike openings and mucous droplets on the surface of tumoral cells.By transmission electron microscopy, sarcomatous cells look like very polymorphous. Secretory cells (showing dilated granular endoplasmic reticulum and abundant lysosomes) are mixed with primitive mesenchymal cells and with intermediate, myoepithelial cells. The myoid origin of these cells is demonstrated by the bundless of myofilaments terminated in marginal plaques on the plasma membrane, the numerous pinocytic vesicles and basal lamina investing irregularly the cells. Histoenzymology corroborates these findings, disclosing a high activity of alkaline phosphatase. Discovery of ambiguous myoepithelial cells associated with undifferentiated cells, likewise reported in epitheliomas, is of a great histogenetic interest. It suggests development of mammary neoplasm from stem cells coming under local of general influences, unknown today, but perhaps responsible of epithelial or conjunctive differentiation. The presence of a contralateral carcinoma in our case is consistent with this hypothesis.