9 medulloblastomas were investigated by electron microscopy. They all showed a rather unique histological and cytological structure. The cells were differently shaped, had in general many cytoplasmic processes and only few organells. Special differentiations which would have made tumour elements look like glial or neural cells were not observed when the tissue samples under investigation were carefully selected by light microscopical examination. Glial or neural elements were found only in infiltration zones where no clear distinction between pre-existing cerebellar and tumourous tissues could be made by conventional histological investigation. Medulloblastomas have essentially the same appearance as parvicellular sarcomas or embryonic tissues. Their common characteristic, viz. that they usually show no special differentiation, is also the most specific quality of medulloblastomas.