We have determined the incidence of hepatitis B surface antigen (HBsAg) in an urban (Enugu) and a rural (Okpatu) population groups in Nigeria. There were no statistically significant differences between the incidence in the two populations. Contrary to earlier reports we did not observe any significant age-related differences in the incidence rates within either population group. In general our results are similar to those reported for the urban inhabitants of Ibadan (several hundred miles west of Enugu) and its surrounding rural population. The low carrier rate recorded for schoolchildren drawn from a mainly high socio-economic group in our population has led us to suggest that socio-economic status alone or together with exposure to mosquito and other insect bites are probably the most relevant factors in the transmission of HBsAg in Nigerians.