Medulloblastomas, originating from cells of the internal granular layer of the cerebellum, developed in Syrian golden hamsters 3--6 months after inoculation at birth intracerebrally and sc with JC virus, a papovavirus isolated originally from a human case of the demyelinating disease progressive multifocal leukoencephalopathy. The lesion is compared with the human medulloblastoma of childhood. JC virus is the first infectious agent to produce such a neoplasm in any species.