We report on the histopathological changes occurring in the thymus of coho salmon Oncorhynchus kisutch experimentally infected with Renibacterium salmoninarum. Coho salmon were intraperitoneally infected, and the thymi were collected weekly from 1 to 7 wk post-infection, and processed for ultrastructural study. The thymus appeared to be infected only in fish collected at 6 and 7 wk post-infection. The first stage of the infection was characterized by the presence of a low number of bacteria scattered in the connective tissue of the capsule. Further progression of the thymic infection was characterized by the rupture of the capsular-thymic barrier, and by the bacterial colonization of the subcapsular and inner zones of the parenchyma, where the bacteria were located mainly in macrophages, but also apparently in pale reticular-epithelial cells and in thymocytes. Other reticular-epithelial cells, and the vascular trabeculae, remained free of R. salmoninarun during the initial stages of the infection. In severely infected thymi, necrosis of the parenchyma of the subcapsular and inner zones, and bacterial invasion of the trabeculae occurred. The integrity of the pharyngeal epithelium covering the thymus was not affected during the infection
