The endocrine pancreas of the freshwater teleost Pimelodus maculatus was studied by electron microscopy. Based on the granule morphology 2 cell types were described: Secretory granules of type I cells are rounded, nearly completely filling the limiting membranous sac which measures from 120 to 150 nm in diameter; the type II granules are also rounded and measure from 220 to 270 nm in diameter; they consist of an eccentrical electron dense core separated from the limiting membrane by a wide electron lucent halo. These characteristics are correlated with those found in other teleosts.