Dactylogyrus intermedius is one of the most common and serious cause of parasitic diseases of freshwater fish in aquaculture, and can cause morbidity and high mortality in most species of freshwater fish worldwide. To attempt controlling this parasite and explore novel potential antiparasitic agents, the present study was designed to ascertain the anthelmintic activity of Chelidonium majus L. whole plant and to isolate and characterize the active constituents against D. intermedius. The ethanol extract from C. majus whole plant showed significant anthelmintic activity against D. intermedius [EC(50) (median effective concentration) value = 71.5 mg L(-1)] and therefore subjected to further isolation and purification using various chromatographic techniques. A quaternary benzo[c]phenanthridine alkaloid exhibited significant activity against D. intermedius was obtained and identified as chelidonine. In vivo anthelmintic efficacy tests exhibited that chelidonine was 100% effective against D. intermedius at a concentration of 0.9 mg L(-1), with EC(50) value of 0.48 mg L(-1) after 48 h of exposure, which is more effective than the positive control, mebendazole (EC(50) value = 1.3 mg L(-1)). In addition, the 48-h median lethal concentration (LC(50)) for chelidonine against the host (Carassius auratus) was 4.54 mg L(-1). The resulting therapeutic index for chelidonine was 9.46. These results provided evidence that chelidonine might be potential sources of new antiparasitic drugs for the control of Dactylogyrus.