A one-year investigation on re-infection rates of opisthorchiasis was carried out in three villages in the Nam Pong Water Resource Development Project, Thailand. In the two villages where selective population chemotherapy was combined with improvements in sanitation and health education aimed at changing food habits, the mean monthly re-infection rate was 2.0% and the annual cumulative rate was 21.5%. In the third village, where only selective population chemotherapy was carried out, the monthly re-infection rate was 5.0% and the annual cumulative rate was 55.5%. Studies on new infections in initially negative individuals were also carried out in the three villages, with inconclusive results.