A cerebellar medulloblastoma from a 2-year-old boy was maintained in vitro in an organ culture system for 6.5 months, and the explants studied by light and electron microscopy at different time intervals. The tumor cells progressively demonstrated divergent differentiation into astrocytes and neuroblasts. Astrocytic differentiation, confirmed by immunohistochemistry for GFA protein, became maximal after about 7 weeks in vitro and was thereafter maintained in different areas of the explants. Concomitantly, neuroblastic differentiation was expressed in other cells, with the progressive development of cell processes filled with many microtubules, of neuroblastic rosettes, of increased numbers of dense-core and clear-centered vesicles, of occasional 9 + 0 cilia, and of synaptic ribbons appearing in vitro. Neuroblastic differentiation was most pronounced in 4- and 6-month-old explants, but synapses were not found. The differentiating features reported are in contrast to those of the original tumor, which was largely undifferentiated. The alternative interpretation of a divergent glial and pineocytic differentiation is also considered. These findings support the concept of the differentiating bipotential of the cerebellar medulloblastoma.