The structural characterization and function of the stomach in the omnivore Pygocentrus nattereri were described using light and scanning electron microscopy. The sac-like stomach was morphologically divided into the cardiac and pyloric regions. The histological structure of the stomach consisted of four layers of the mucosa, submucosa, muscularis, and serosa. The superficial epithelium of the cardiac stomach was lined with columnar epithelial cells and the glandular epithelium contained numerous gastric glands. Gastric glands were completely absent in the pyloric portion. The mucosal surface of the stomach was a meshwork of various folds, provided with oval or rounded columnar epithelial cells which were densely packed with short, stubby microvilli. The occasional presence of conspicuous gastric pits was surrounded by epithelial cells. The localization and chemical nature of acid and neutral mucins in the various cells of the stomach was studied by employing combined the Alcian Blue-Periodic Acid Schiff (AB-PAS) technique. The deposition of glycogen was detected in the gastric glands as well as in the epithelial lining of the stomach. The utmost reactions for protein and tryptophan were recorded in the gastric glands of mucosa. The cellular organization and histochemical characterization of the stomach are discussed in relation to the feeding and digestion of the fish concerned.