A 25 cm adult oscar, Astronotus ocellatus (Cuvier), older than 5 years with abnormal behaviour, abdominal distension and general deterioration of body condition was received from a private owner for evaluation and treatment. The oscar bad been kept in a 200L aquarium, shared with a plecostomus catfish. Water quality parameters were maintained within normal limits for this freshwater tropical cicblid. It had been healthy for years when its condition began to deterio-rate, and abdominal distension became noticeable about 3 months before death.The owner first took the fish to the local pet store and was advised to withhold food for 2 weeks, but this produced no improvement. Lethargy and debilitation progressed, and it began to list over time. When presented to the laboratory for clinical evaluation, 60 mm of dark brown, serosanguinous transudate was aspirated from the caudoventral abdomen. Treatment consisted of the broad spectrum antibiotic chloramphenicol at a dose of 50 mg kg"' i.m. and the an ti-inflammatory drug dexamethasone at a dose of 2.0 mg kg"' i.m. Despite these measures, the fish died approximately 12 h after presentation. Necropsy revealed an enlarged, polycystic posterior kidney, while all other internal organs appeared grossly normal.Sections of the kidney, heart, liver, spleen and stomach were routinely prepared for histology. The posterior kidney sections consisted of multiple