Electroreception evolved in the catfishes probably as a specialization of the mechanosensory lateral line system. Fibers of the anterior lateral line nerve in catfishes innervate electrosensory ampullary organs and mechanosensory neuromasts of the head lateral line system. The purpose of this study is to determine the projection patterns of the major principal branches of the ALLN and to investigate the topography within the various nuclei of the terminal fields of these different branches. Fibers of the superficial ophthalmic, buccal and hyomandibular branches of the anterior lateral line nerve terminate in a somatotopic fashion within medullary and cerebellar nuclei. These fibers project to, and terminate within, several discrete nuclei in the medulla and cerebellum, notably the electrosensory lateral line lobe, the medial and caudal octavolateralis nuclei, and portions of a nuclear complex in the cerebellum called the eminentia granularis. Furthermore, the dorsoventral somatotopy in the medullary electrosensory nucleus is a reversed or 'mirror' image of that in the mechanosensory nucleus. This reversed map is similar to that observed in other electrosensory systems and suggests that there may be a common mechanism for the copying and preservation of spatial information as new systems are evolved from primitive sensory pathways.