Although it has long been accepted that the structure of the cerebellar cortex is the same all over its areas, some of the recent studies indicated that there are regional differences in the time course of development of the cortical layer structures as well as in its cytoarchitecture. The cerebellum consists of 3 phylogenetically different areas; archi-, paleo- and neocerebellum, which deals with vestibular, spinal cord and cerebral cortical activities respectively. The development of the structures of cerebellar cortex occurs mostly after birth in the altrical animals, and the control of the posture and movement takes place faster in the trunk than in the extremities in such animals. It could be expected that the postnatal development occurs faster in the phylogenetically older parts than in the newer ones. To elucidate the above mentioned assumption, post-natal development of the cerebellar cortical structures was studied, cytoarchitectonically and cell morphologically, using cats from birth to the adult state. The external granular layer increased in thickness from birth to 2-9th postnatal days, then decreased gradually, and almost disappeared by the 63rd postnatal day. While the molecular and the internal granular layers increased in thickness gradually after birth, and reached the adult level by the 63rd postnatal day. Cytoarchitectonically, postnatal development took place faster in the archicerebellum, next in the paleocerebellum and slowest in the neocerebellum. Silver impregnation study showed the first dendritic arborization of the Purkinje cell appeared on the 7th postnatal day in the arch- and paleocerebellum, while the same phenomenon was observed on the 14th postnatal day in the neocerebellum. The above observations confirmed our assumption that phylogenetically older parts of the cerebellar cortex have faster postnatal development than the newer ones. This is in agreement with the fact that the function of the neuronal activity in these phylogenetically older parts occurs earlier than in the newer parts.