Oligodendrocytes (OLs) are the glial cells of the central nervous system and are classically known to form myelin sheaths around most axons of higher vertebrates. Whether these cells might have other roles, in particular during development, has not been studied. Taking advantage of a transgenic mouse model in which OLs can be selectively killed in a desired time-frame, we have investigated the impact of OL ablation on cerebellar development. OL ablation was induced during the first 3 postnatal weeks, a time at which cerebellum development is ongoing. Strikingly, OL ablation triggers a profound perturbation of the known cerebellum developmental program, characterized by the disorganization of the cortical layers, abnormal foliation and a complete alteration of Purkinje cell dendritic arborization and axonal fasciculation. This phenotype is accompained by decreased granule cell density, a disorganized Bergmann glia network and impaired migration of interneurons in the molecular layer. These results demonstrate a previously ignored role of OLs in the formation of the cerebellar cytoarchitecture.