Incidence, measured as the proportion of persons whose stools become positive within one year, was studied in endemic Opisthorchis viverrini, the human liver fluke, in a northeastern Thai village over a two-year period. Incidence was higher in males than in females, especially in children under five years of age. It was at least 47% overall in the first year of the study, but declined to below 20% per year in the second. This is attributed to drying of a local water reservoir and decline in availability of infective stages in fish. The fluctuation of incidence is probably due to the large variations in rainfall from year to year. The rate of reversion from positive to negative varied from 2% to 6% per year.