Electron microscopic observation was made on 2 specimens of Ewing's sarcoma. The lesion was found mainly composed of light cells and a few dark cells. The former were round and polygonal in shape, containing large amounts of glycogen. Relatively few organnellae were seen in the cytoplasm. Many of these cells showed conjugation with small desmosome-like structures. The latter had irregular projections which appeared as isolated unities and seemingly degenerated structures can be found in between the light cells. Clusters of special dense granules can be seen within the nucleus and virus like inclusion particles were found in the cytoplasm; the signficance of both remains to be clarified. The presence of abundant glycogen inside these cells may help to distinguish this tumor from reticulum cell sarcoma and metastatic neuroblastoma.