In the course of ultrastructural studies on the cerebellar cortex of young rats a cilium was found in some Purkinje cells. The shaft, which is 1800 to 2000 ÅR thick, has an internal portion inside the cell body and an external one projecting into the neuropile. The basal body is consistently located near the Golgi apparatus. Rootlets with periodic cross-striations (interperiod distance of 525 ÅR) are attached to the basal body. The size and characteristics of the Purkinje cell cilium and its constituent parts compare with similar structures in the cilia of other cell types of the cerebellar cortex. Whether all or only some of the Purkinje cells have a cilium has not yet been determined. Possible functional roles of these organoids are considered.