The posterior lateral line lobe of the wave species of gymnotoid fish was investigated with the Golgi technique. The posterior lobe has a laminar structure and contains II cell types differentially distributed in the varous laminae (fig. 13). The major laminae, from ventral to dorsal are the deep fiber layer, containing multipolar neurons; the deep neuropil layer, containing ovoid neurons and a sub-lamina of spherical cells; the granule cell lamina, containing two types of granule cell; the plexiform laminae; the polymorphic cell lamina, containing basilar pyramids, non-basilar pyramids, giant fusiform cells, and polymorphic cells; the stratum fibrosum; the molecular lamina, containing neurons of the ventral molecule layer and stellate cells. The spherical cells are regularly distributed in their sub-lamina and appear to receive one type of primary afferent input. Another type of primary afferent input ends in the deep neuropil and granule layers, in proximity to the basilar dendrites of the granule cells and the basilar pyramids. The basilar pyramidal cell spatially alternates with the non-basilar pyramidal cell, so that the basilar dendritic trees of nearest-neighbour basilar pyramids show almost no overlap. Descending input to the posterior lobe ends in the molecular layer, in proximity to apical dendrites of both pyramidal cells, giant fusiform cells, polymorphic cells, and one type of granule cell. There are three afferent fiber systems in the molecular layer, one running transversely, one longitudinally, and one vertically. Local circuity in the posterior lobe is precisely organized and involves projections of granule cells onto overlying pyramidal cells. The polymorphic cell may also be involved in the intrinsic circuits of the posterior lobe.