Injections of horseradish peroxidase into the optic tectum of the piranha resulted in retrograde transport to the following structures bilaterally: the central telencephalic nucleus, four hypothalamic nuclei, the caudal part of the dorsomedial optic nucleus, the ventral nucleus of the torus semicircularis, the torus longitudinalis, the perilemniscal nuclei, the reticular formation, and a tentatively identified locus coeruleus. In addition, labeled cells were found unilaterally in the contralateral tectum and in the medial octavolateralis nucleus, the ipsilateral thalamic portion of the dorsomedial optic nucleus, the corpus geniculatum laterale ipsum, the pretectal nucleus, the nucleus corticalis, the dorsal nucleus of the torus semicircularis, and the nucleus isthmi. Efferent projections of the optic tectum appeared identical to those reported in Holocentrus (Ebbesson and Vanegas 1976).