Flubendazole is a veterinary anthelmintic which is used in Europe to treat intestinal nematodes in horses and companion animals. The compound has not been approved by the Food and Drug Administration for sale in the United States. Despite its lack of FDA approval, the compound has become popular among tropical fish hobbyists for control of Capillaria sp. in discus (Symphysodon sp.), based on recommendations of European publications. On two separate occasions, discus has been submitted to the aquatic animal diagnostic laboratory at the University of Florida with signs of liver disease. In one case, the animals were severely jaundiced and showing behavioral signs suggestive of hepatic encephalopathy. Histologic examination of liver tissue revealed severe toxic hepatitis. A clinical study was conducted at the University of Florida to assess the potential toxic effect of flubendazole on discus. A second clinical trial was conducted to assess efficacy of the compound as a treatment for Capillaria sp., using angel fish (Pterophyllum scalare) as a model.