In a longitudinal study of a malaria-endemic village in southeastern Thailand, circumsporozoite (CS) antibody to sporozoites of Plasmodium falciparum was measured by an enzyme-linked immunosorbent assay to determine its usefulness as a seroepidemiologic marker of malaria transmission. The CS anti-(NANP)n antibody level and prevalence during a 25-month period paralleled the pattern of seasonal transmission consistent with conventional parasitologic and entomologic measurements. The prevalence and level of antibody decreased during the non-transmission wet season, and increased over a 1-2-month transition period between the end of monsoon rains and the onset of dry conditions, an interval of maximum vector activity. Antibody increased with age in the population. The prevalence of antibody to the asexual blood stage as measured by conventional indirect fluorescent antibody assay did not coincide with changes in transmission and was sustained throughout the study period. Thus, CS antibody appeared to reflect the relative population exposure to mosquito inoculation of P. falciparum sporozoites and provided a useful measure of malaria transmission dynamics.