In the present study, the techniques of histological section and immunohistochemistry were adopted to perform a histological observation on the pancreas of largemouth bass. It was revealed by the sections stained with H-E and G-aldehyde fuchsine that more than 10 yellow-white macroscopic nodules were identified as islet structures in the abdominal cavity of largemouth bass, in which the biggest one was designated as principal islet, and a lot of smaller invisible islets were embedded in the wall of the common bile duct. The islet was surrounded by an inner layer of connective tissue and an outer layer of exocrine pancreatic tissue, and the exocrine pancreatic tissue also existed and was distributed in the liver. The sections marked by streptavidin peroxidase (S-P) immunohistochemical method and the section image analysis suggested that B cells were located mainly in the central region of the islet while the A cells were distributed throughout the islet. It can be concluded that the pancreas of largemouth bass belongs to dispersal type, the endocrine pancreas (islet) is distributed mainly around the common bile duct while the exocrine pancreas surrounds the islet and it is also embedded in the liver.