This report describes the histologic, immunologic, and ultrastructural features of a distinctive spindle-cell tumor of the female breast interpreted as pure myoepithelioma. By light microscopy, the tumor showed the mammary parenchyma replaced by bundles of fusiform cells, which cytoplasms contained myosin and actin, demonstrated immunologically. Ultrastructurally, the spindle cells were joined by mature desmosomes and presented parallel bundles of microfilaments and remnants of basal lamina.