A liver fluke, Opisthorchis viverrrini (OV), is the major cause of the high incidence of cholangiocarcinoma in Thailand. The prevalence of OV infection remains high in various parts of the country, especially in Northeast Thailand and particularly in wetland rural areas where a large proportion of the community work in agricultureand continue the traditional practice of eating raw or uncooked cyprinoid fish products. The national control program seems to have had little impact in many of these areas, and it has been difficult to make precise assessments of the overall effectiveness of the program. This paper is the first report of prospective research project designed to monitor the impact of the national control program in rural communities located in a northeastern province and at high risk of OV infection. The participants in this initial survey were 1,569 villagers, aged 20-65 years, living in two subdistricts of Yasothon Province. Stool examinations showed that 38.68% were infected with OV. Males were slightly more likely to be infected than females, but the difference was not statistically significant. Infection was found to be positively associated with age in both males and females. The preliminary data indicate that the population selected for study is suitable for the purpose of the monitoring project.