The accessory optic system of Rana pipiens consists of lateral and medial fascicles of the basal optic root (BOR1, BORm) and a single terminal nucleus, nBOR. The present study provides new evidence that these two fascicles differ not only in their trajectories but in fiber spectra and innervation patterns as well. BOR1 contains a substantially higher percentage of large, myelinated axons than does BORm, and the mean diameter of axons in BOR1 is greater than that of BORm. BOR1 innervates the entire terminal field of nBOR, while BORm innervates only the central and mediodorsal portions of nBOR. The ventrolateral portion of nBOR is uniquely innervated by BOR1 and contains several types of neurons not found in the central and medial regions of nBOR which are innervated by both fascicles. Cytoarchitectural analysis of nBOR with Golgi techniques has revealed a number of similarities between the anuran nBOR and the mammalian medial terminal nucleus (MTN) with regard to cellular morphology, dendritic geometry, and retinofugal arborization patterns. In frog, nBOR appears comparable to the ventral subdivision of the mammalian MTN, while the peri-nBOR region, which contains neurons postsynaptic to nBOR, may represent a more primordial version of the mammalian dorsal MTN.