We conducted a seroepidemiologic study of hepatitis B virus (HBV) infection among 14,198 Inuit (Eskimo), Dene (Indian), and non-native inhabitants of the Northwest Territories (NWT, total 1982 population 47,053) between April 1983 and March 1985. Participants represented almost all of the NWT communities and included approximately half of the total indigenous population. Sera were screened for hepatitis B surface antigen (HBsAg) and antibody to hepatitis B surface antigen (anti-HBs) by radioimmunoassay. A total of 428 persons (3.0%) were positive for HBsAg; their mean age was 41.5 years and 68.7% were greater than 30 years old. Anti-HBs was detected concomitantly in 153 (35.7%) of these persons and present as a single marker in 2,943 sera (20.7%). Overall prevalence rates for HBsAg were marginally higher among Inuit than Dene (3.9% and 2.9%, respectively; P less than 0.05), as were rates for anti-HBs (24.5% and 21.5%, respectively; P less than 0.01). Marker rates among the 2,776 participants of other ethnic groups, predominantly whites, were distinctly lower (HBsAg 0.3%, anti-HBs 8.5%) than among Inuit or Dene populations (P less than 0.01). The prevalence of HBsAg and anti-HBs increased with each decade of life in all five geographic regions, but there were marked differences among the regions as well as among communities within regions. The greatest concentration of HBV markers was in the Baffin Region (36.0% positive for HBsAg or anti-HBs). Hepatitis B e antigen (HBeAg) was detected in only 37 of 421 persons (8.8%) positive for HBsAg; most were less than or equal to 20 years old and were clustered in a few communities with the highest HBV marker rates.