Ten cases of human cerebellar medulloblastomas were examined by light and electron microscopy. Ultrastructural differentiation of tumor cells was discussed in relation to the histological patterns. In areas which appeared densely cellular with light microscopy, the tumor was composed of a series of undifferentiated cells having a variety of junctions between tumor cells. In areas showing rosette formation with light microscopy, many elongated cytoplasmic processes and numerous mutually entangled intracytoplasmic microtubules were seen in the center of Homer-Wright pseudorosettes. In the area seen as a whorled pattern by light microscopy, elongated fine cytoplasmic processes showing a lamellar array were prominent. In the spongioblastic parts with PTAH-positive fibrils, intracytoplasmic glial filaments were not observed by electron microscopy even though a few fine fibrillary structures were present. The formation of solitary cilia was observed only in the desmoplastic area by light microscopy. Therefore, the medulloblastoma was composed of extremely undifferentiated cells which only a little neuroblastic differentiation. This was limited to the rosette forming areas.