Serum samples were non-randomly obtained from apparently healthy inhabitants of 5 villages in the Amazonas State, Brazil. Sera were tested by enzyme immunoassay for the presence of hepatitis B surface antigen (HBsAg) and, if this was found, for antibody to hepatitis delta virus (HDV). Of 574 people tested, 96 (16.7%) were reactive for HBsAg and 33 of these (34.4%) for anti-HDV. The results were analysed according to sex, age and history of jaundice and showed a high prevalence of hepatitis B virus (HBV) and HDV infections in these asymptomatic persons, mainly in young people. The rates of prevalence observed in Amazonas for both HBV and HDV are perhaps among the highest in the world, demonstrating that these viruses are endemic in this region of Brazil.