Dead oscars (Astronotus ocellatus) were presented for necropsy in January, 2005. Oscars, which had been reared densely, were dead in large quantities. At necropsy, they were severely infected by Dactylogyrus spp. We requested for the oscar samples which had been raised in the same water tank. Two of them were sacrificed and necropsied. At the microscopic examination, we verified dactylogyrosis of oscars. The other oscars were treated by praziquantel. Treatment was started with a dosage of 2.5 mg/L. From a second day, we treated 5.0 mg/L every two days during 4 days. At sixth day, No Dactylogyrus flukes were found but only larvae were existed. After 2 weeks without additional medication, both Dactylogyrus flukes and larvae were eliminated.