Dendritic spines have been investigated intensively over recent years; however, little is yet known about how they organize on the cell surface to make synaptic contacts with appropriate axons. Here we investigate spine distributions along the distal dendrites of cerebellar Purkinje cells, after biolistic labeling of intact tissue with a lipid-soluble dye. We show that the spines have a preference to form regular linear arrays and to trace short-pitch helical paths. The helical ordering is not determined by external factors that may influence how individual spines develop, because the same periodicities were present in fish and mammalian Purkinje cells, including those of weaver mice, which are depleted of the normal presynaptic partners for the spines. The ordering, therefore, is most likely an inherent property of the dendrite. Image reconstruction of dendrites from the different tissues showed that the helical spine distributions invariably lead to approximately equal sampling of surrounding space by the spineheads. The purpose of this organization may therefore be to maximize the opportunity of different spines to interact with different axons.