This study was conducted in an Afro-Brazilian, slave-descendant community with high (42.4%) hepatitis B virus (HBV) prevalence. Twenty (8.4%) out of the 239 subjects under study were HBsAg-positive, and HBV-DNA was detected in 59 (25%) individuals. A high rate (18.3%) of occult infection was therefore observed that was associated to low HBV loads (mean, 1.8 x 10(4) copies/ml) and to a specific amino acid substitution (C100Y) in the small surface antigen. Genotyping of 50 isolates showed that 43 (86%) were of subgenotype A1, one (2%) from subgenotype A2, and five (10%) from subgenotype D. Mixed genotypes A1 and E were observed in one (2%) sample. The genetic distance (0.8 +/- 0.3%) among the HBV/A1 isolates from the community was smaller than the intragroup divergence among A1 isolates from Brazil as a whole, but it was similar to that found between A2 isolates from different countries, suggesting that HBV/A1 was introduced in the community through different sources. The substitution W501R (polymerase), previously reported only in Gambia, was observed in 46% of the HBV/A1 isolates. The precore/core promoter region of HBsAg-positive isolates showed several substitutions that could explain the anti-HBe phenotype found in 18 of 20 (90%) of the HBsAg-positive subjects.