No recent data are available on hepatitis B virus (HBV) and hepatitis Delta virus (HDV) prevalence in Mauritania. One thousand twenty pregnant women and 946 patients visiting for routine checkups were screened for HBV and HDV infection. Demographic, epidemiological, ethnic, clinical, and biological data were recorded. HBV and HDV genotypes were determined by sequencing and phylogenetic analyses. In the pregnant women and patients cohorts, respectively, the prevalence of HBsAg (10.7% and 18.3%) and anti-HBcAb (66.3% and 76.5%) indicated high HBV endemicity. In pregnant women, exposure to HBV was significantly associated in multivariate analysis with education level, ethnicity, blood transfusion, and occupation. HDV antibodies (HDVAb) were found in 14.7% of pregnant women. In patients, HBsAg was found less frequently in females than in males. Again in multivariate analysis, exposure to HBV was significantly correlated with gender (males), and HDVAb positivity with age and gender. The HBV DNA viral load was >3 log IU/ml in only 10.1% of pregnant women and in 17.3% of patients. HDV-RNA was detectable in 21 (67.7%) of the 31 patients positive for HDVAb, and in 11 of the 16 pregnant women positive for HDVAb (68.8%). The most frequent HBV genotypes were: HBV/D, 53%; HBV/E, 35%; and HBV/A, 12%. Sub-genotyping revealed HBV/D1,/D7, and the recently described/D8. HDV genotypes were: HDV-1, 90.3% and HDV-5, 9.7%. This study confirms the high prevalence of HBV and HDV infections in Mauritania and demonstrates the high genetic diversity of HBV in this country. J. Med. Virol. 84: 1186–1198, 2012. © 2012 Wiley Periodicals, Inc.