The case of an infant with a peculiar tumorous malformation of the cerebellum is described. The tumor apparently developed as an exophytic, hypertrophic sprout of the inferior vermis. It had a monstrous appearance resembling a crab, with a metameric body and multiple pairs of limbs attached to the folia of both cerebellar hemispheres. Histologically, the lesion was formed by poorly differentiated neuroepithelial cells without any evidence of organization into nuclei, cortex, or fascicles. Clinically, the tumor behaved in indolent manner and did not regrow after subtotal surgical resection. Because of its gross appearance and its biological behavior, this unusual hamartoblastomatous growth is readily distinguished from medulloblastoma. The morphology of the cerebellum in Lhermitte-Duclos disease is reviewed, and a new interpretation of its basic structure is proposed. This and other known types of cerebellar hypertrophy are different from the malformation in the present case.