The Stannius corpuscles of Prochilodus scrofa are studied microscopically (both optically and electronically) as well as macroscopically. Two CS, and sometimes 3 or 4, are found per animal. They are pink-coloured organs, rounded in shape and scattered laterally in the first anterior third of the kidney. Each CS is enveloped by a thick connective tissue capsule. The grandular parenchyma consists of only one cellular (PAS-/AF-) type, cylindrical in shape, with oval or rounded nucleus located eccentrically with visible nucleolus. The ultrastructural description is as follows: well developed Golgi complexes, rough endoplasmic reticulum and mitochondria in moderate quantities. Electron-dense granules numerous. CS highly vascularized.