The caudal lobe of the cerebellum of the high frequency gymnotid fish Apteronotus leptorhynchus is that region of the cerebellum lying lateral to the posterolateral sulcus. It consists of three granular masses--the eminentia granularis posterior pars lateralis, a transitional zone T, and the eminentia granularis posterior pars medialis--with their associated molecular layers. We have used the retrograde transport of wheat germ agglutinin conjugated horseradish peroxidase to study the afferent input to the various subdivisions of the caudal lobe. Each granular mass receives different types of input. Eminentia granularis posterior pars lateralis receives a massive bilateral input from an isthmic nucleus, nucleus praeeminentialis, concerned with descending control of the electrosensory system and from a rhombencephalic nucleus, the lateral reticular nucleus, which itself receives a major spinal input. In addition eminentia granularis posterior receives lesser input from other pretectal, (N. at base of dorsomedial optic tract, pretectal complex "B") mesencephalic (dorsal tegmental N., nucleus raphe dorsalis), isthmic (bed N. of praeeminentialis-cerebellaris tract, locus coeruleus) and rhombencephalic nuclei (lateral tegmental N., eurydendroid cells, octaval N., perihypoglossal N., paramedian reticular N., medullary reticular formation, medullary raphe, efferent octavolateralis N., inferior olive, and funicular N.). The input from nucleus praeeminentialis dorsalis is mapped topographically onto eminentia granularis posterior with respect to their rostro-caudal location. We could not define any topography in the mapping of the dorso-ventral body axis upon eminentia granularis posterior; small injections of WGA-HRP produced several small clusters of labeled cells within nucleus praeeminentialis dorsalis which does suggest a more complex organization of this projection. Zone T receives most of its input from the ipsilateral VIIIth nerve ganglion cells and certain pretectal nuclei, but it also receives a small input from nucleus praeeminentialis dorsalis. Eminentia granularis posterior pars medialis receives minor input from a small pretectal nucleus and a small ventral diencephalic nucleus, this region appears to receive its major input from eurydendroid cells of eminentia granularis posterior. The molecular layer associated with each granular mass receives contralateral input from separate clusters of inferior olivary cells. In addition the eurydendroid cells (cerebellar output neurons) of eminentia granularis posterior pars lateralis receive a substantial direct input from cells located in the medial aspect of nucleus praeeminentialis dorsalis.