OBJECTIVE: To estimate the prevalence of HIV and associated sociodemographic factors including mobility and migration in a rural population in KwaZulu-Natal, South Africa. METHODS: A household-based HIV serosurvey of a population that has been under longitudinal demographic surveillance since 2000. All residents (women aged 15-49 years; men aged 15-54 years) and a sample of non-residents ('migrants') who return periodically to their households in the area were identified and approached for finger-prick HIV testing. RESULTS: A total of 8325/11 505 male and 11 542/14 396 female residents were traced. Of these, 4692 men and 6859 women consented to HIV testing. Overall, 27% of female and 13.5% of male residents were HIV infected. HIV prevalence peaked at 51% among resident women aged 25-29 years and 44% among resident men aged 30-34 years, with the highest infection rates of 57.5% among 26-year-old women. The female: male infection ratio for residents aged 15-19 years was 13.0. Many factors, including increased mobility, associated with an increased risk of HIV infection among residents, were also associated with non-participation. Among non-residents, 34% of men aged 15-54 years and 41% of women aged 15-49 years were HIV infected. CONCLUSION: The extremely high prevalence of HIV suggests an urgent need to allocate adequate resources for HIV prevention and treatment in rural areas. Effective monitoring of the epidemic in Africa needs to include efforts to strengthen sentinel surveillance in rural areas and strategies for the surveillance of migrants and mobile individuals.