Serum samples collected from two groups in the Tari District of Southern Highlands Province were assayed for markers of hepatitis B virus (HBV) infection. 85% of women of childbearing age were found to have markers of HBV infection; 37% were positive for HBV surface antigen (HBsAg), indicative of the chronic carrier state, and 6.6% were positive for HBV e antigen (HBeAg), indicating the presence of actively replicating virus. 75% of women negative for HBsAg were positive for antibody to HBV core antigen (HBcAb), a marker of past infection. A group of children aged 6 to 18 years showed a significantly lower prevalence of markers of infection (66%) but higher rates of HBsAg positivity (46%) and HBeAg positivity (30%). Only 37% of the HBsAg-negative children in this group were positive for HBcAb. The results from this serosurvey suggest that the major route of HBV transmission in this population is horizontal, between older children, though significant transmission also occurs during the neonatal period.