In a pilot control project of Opisthorchis viverrini in the village of Nong Wai Irrigation Area, Khon Kaen, Northeast Thailand, selected population based chemotherapy with a single dose of 40 mg/kg of praziquantel (2 - cyclohexylcarbonyl-1,2,3,6,7,11 b-hexahydro-4H-pyrazino [2,1-a]isoquinolin-4-one,EMBAY8440,Biltricide) was given to the stool positive cases once a year for a period of 3 years commencing May 1981. Two villages (pop 942) with a prevalence of 58.3% were used as tests villages and another one (pop. 442) with a prevalence of 55.1% served as control. The project programme integrated chemotherapy, sanitation improvement and health education, whilst only chemotherapy was applied in the control village. Among the 736 cases of all villages, 666 were treated in the first year and the parasitological cure rate after one month was 95.9%. Side effects of praziquantel were minor and were minimized by changing the time of drug intake from morning to after dinner. After one year the prevalence of opisthorchiasis was 36.8% in the test villages and 54.8% in the control village. The study on re-infection rate revealed that the average monthly incidence rate among the population in the test villages was 2.0% per month (range 1.1-5.0%) whereas it was 5.0% per month (range 1.9-10.3%) in the control village. In May 1982, second treatment was given again to the positive cases in the three villages with the same dose. The second year evaluation is in progress.