The organization of the cerebellar nucleocortical projection in the cat and the monkey has been studied using orthograde and retrograde neuroanatomical tracing techniques. Injections of tritiated leucine in the cat cerebellar nuclei orthogradely labeled neucleocortical fibers throughout their course to the cerebellar cortex. Their branch points in the corpus medullare, in the folial white matter, and in the granular layer were evident from the dense, continuous distribution of silver grains overlying these labeled axons. The results from the cat showed that the cerebellar nucleocortical projection is organized principally into three rostrocaudally oriented longitudinal cortical zones. Fastigial nucleocortical fibers were directed principally to the medial 1.5–2.5 mm of the ipsilateral vermis, with a lighter projection to the lateral vermis ipsilaterally and to the medial area of the vermis contralaterally. The interposed nuclei projected mainly to the paravermis-medial hemispheric zone of the cerebellar cortex. Nucleocortical fibers from the posterior interposed nucleus projected principally to the paramedian lobule, to the medial hemispheric area of Crus I and the lobus simplex, and to the flocculus and paraflocculus. Nucleocortical projections from the anterior interposed nucleus coursed to the anterior lobe paravermis and to the ventral folia of the paramedian lobule. A lighter projection from the interposed nuclei was found to the lateral edge of the vermis and into intermediate areas of the hemisphere. Dentatocortical fibers were directed into the lateral folia of Crus I and Crus II of the lateral hemispheric zone, with a lighter projection to intermediate areas of the hemisphere of the posterior lobe and along the lateral edge of the anterior lobe hemisphere. Along the periphery of each cortical zone, the nucleocortical projection from adjacent deep nuclei overlapped slightly. The retrograde transport of horseradish peroxidase (HRP) from injection sites in the lateral hemisphere, in the medial hemisphere — paravermis, and in the vermis labeled neurons localized mainly within the dentate, interposed, and fastigial nuclei, respectively. Retrograde labeling experiments carried out in monkeys indicated that the organization of the neucleocortical projection in this species is different than that of the cat. In the primate, the nucleocortical projection to the lateral hemisphere, to the medial hemisphere — paravermis, and to the vermis appeared to arise principally from the dentate nucleus. There was a secondary input to the paravermis and vermis arising from the interposed and fastigial nuclei, respectively. This evidence suggests that the cerebellar nucleocortical system undergoes a significant phylogenetic change in its organization between the cat and primate. These organizational differences are discussed in light of possible functional implicatons.