A total of 919 hepatitis B surface antigen (HBsAg) positive serum samples from Kyushu and Okinawa, Japan, were subtyped for the d, y, w, and r specificities. There was a great difference in distribution of subtypes between Kyushu, where over 70% of the samples were adr, and Okinawa, where over 70% were adw. Among 306 members of 109 families with two or more hepatitis B virus carriers, HBsAg subtype was always identical within each family, even when there had been close contact between members of families with different subtypes. These results suggest that transmission of hepatitis B virus occurred within families rather than between members of different families, even when there was close contact between members of different families.