In a seroepidemiological study of hepatitis B virus (HBV) over 7 years from 1982 to 1988, in a fixed population of 633 healthy students (15-20 years old), it was found that one HBV carrier who had adw subtype of HBV had also become sero-positive to r subtype, suggesting the superinfection with r subtype of HBV to the carrier. Then, some serological markers of HBV were further examind in the other 10 HBsAg positive carriers in the same population, and discussed the possibility of horizontal transmisison of r subtype of HBV in the pupulation. Of the 10 HBsAg positive carriers, 5 possessed adr subtype, 2 adwr, 2 adw, respectively, and the remaining one was not tested. Four of seven r subtype of HBsAg positive carriers exhibited pre-S2Ag and HBeAg activities which were considered as infectious markers. Epidemiological survey was carried on the four carriers with special refference to the possibility of the superinfection. As a result, it is still unclear the source of the superinfection, but at least, iatrogenically accidental transmission at the time of vaccination and contact infection in everyday life in school may be unlikely to attribute to the horizontal transmission.