An epidemiological survey of the prevalence of hepatitis B virus (HBV) was conducted in the county of Nossa Senhora do Livramento, Mato Grosso state, central Brazil. This rural county has a largely stationary population of low socio-economic status, and is divided into savannah-type vegetation and wetland. Overall, 740 subjects aged > 9 years were selected at random, interviewed and bled so that seropositivities of various HBV markers (surface antigen and antibodies to this antigen and to core antigen) could be determined. At least one marker was found in 169 (22.8%) of the subjects but only nine (1.2%) carried the surface antigen. Vegetation type (wetland v. savannah) was not associated with HBV infection but male gender, increasing age, and having had sexual experience were each associated with the infection. The prevalence of HBV markers was higher in immigrants (32.5%) than in native individuals (21.1%), many immigrants apparently having acquired the infection before settling in the study area.