The efficacy of two active compounds, isolated from the fruit of Fructus cnidii, was determined against Dactylogyrus intermedius in goldfish (Carassius auratus). The chemical structure of the two compounds was identified by spectral analysis. Designated Compounds I and II, they were isolated by successive silica gel column chromatography and used, in combination, in an in vivo anthelmintic efficacy assay. At dose rates of 1.6 and 9.5 mg l(-1), respectively, the two compounds were 100% effective against D. intermedius compared to mebendazole (positive control), used at a concentration of 1.5 mg l(-1), which is 100% efficacious. Compounds I and II were shown to be safe for goldfish at dose rates of up to 6.4 and 12.5 mg l(-1), respectively. The median effective dose (ED(50)) of Compounds I and II for D. intermedius-infested goldfish after 48 h exposure was 0.807 and 8.050 mg l(-1), respectively. If the median lethal dose (LD(50)) values for goldfish of the two compounds are 6.749 and 12.759 mg l(-1), respectively, the ED(50) values for D. intermedius yielded a therapeutic index of 8.14 and 1.52 for Compounds I and II, respectively, at 48 h. Compounds I and II were identified as osthol and isopimpinellin by physico-chemical detection.