This case report describes the radiographic, ultrasonographic and computed tomographic (CT) examination of an abdominal swelling in a Red Oscar (Astronotus ocellatus Cuvier, 1829). While radiography only demonstrated caudoventral compression of the swimbladder, ultrasonography found a huge tumor-like parenchyma in the caudal abdomen. CT studies detected a possible connection between the tumor and the kidneys. Necropsy and pathohistology were performed. A papillary-cystic adenoma of the kidney was diagnosed.