In an attempt to analyze the clonorchicidal activity of linoleic acid and ethyl linoleate in vitro, the wormicidal effects on Clonorchis sinensis were chronologically monitored in dose titration experiments. Excysted metacercariae were killed within a period of 31.0+-4.0 min, 149.3+-4.l min and 207.0+-13.5 min with 100.0 mg, 0.1 mg and 0.001 mg linoleic acid, respectively. The time required for the linoleic acid to kill adult worms was 167.0+-0.8 min with 100.0 mg, 253.0+-0.8 min with 0.1 mg, and 277.0+-0.8 min at 0.001 mg titration. Clonorchicidal activity of ethyl linoleate was relatively delayed as death was observed within 263.3+-2.9 min, 286.0+-0.5 min, and 318.0+-0.8 min for 100.0 mg/ml, 0.1 mg/ml and 0.001 mg/ml concentrations, respectively. The wormicidal effects observed with these pure anticlonorchal substances were found to be similar to the biological activity of native products derived from the mucus of the fresh water fish.