A histopathological study was made on Pseudomonas fluorescens infection which occurred in the spring of 1979 in pond-cultured tilapia, Sarotherodon niloticus. The infected fish showed exophthalmia, dark body coloration, spotty or nodular lesions in the liver, spleen, kidney and gills, and inflamed swim-bladder. The bacterial dissemination was systemic. The histopathological aspects were abscess formation in eyes, spleen and swim-bladder, and focal necrosis in the liver, gills and kidney of some diseased fish. The other fish showed granuloma formation in all infected lesions.