This paper describes patterns of infection with Opisthorchis viverrini within a human community assessed by egg count, parasite-specific antibody levels and worm burden determined by expulsion chemotherapy. All three measurements increased significantly with age, reaching a plateau in young adults. There were no significant sex-associated differences in the prevalence or intensity of infection, antibody levels, or in the frequency of heavy worm burdens. A striking 81.5% of the total Opisthorchis population and 74% of the total egg output were expelled by the most heavily infected 10% of the humans sampled. This extreme over-dispersion, together with recent findings that hepatobiliary disease frequently accompanies heavy infections, argues that the priority for community-based parasite control programmes should be to maximize treatment and health promotion efforts toward heavily infected individuals.