Objective: To confirm drug resistant hepatitis B virus mutants frequently appear among drug intravenous user (IDU) and female sex worker (FSW) groups in Haiphong, Vietnam. Subject and methods: Plasma samples were collected from 633 subjects among two groups (IDU: 492
FSW: 141) in the Rehabilitation Center in Haiphong. Plasma viral DNA was extracted from samples that were positive for hepatitis B suface antigen (HBsAg). Pol-RT regions of HBV were amplified by PCR with primers (HB1F(18-39)/HB1R(534-557) and HB2F(414-433)/HB2R(970-989)). PCR products were sequenced directly and analized by GENTYX program, introduction of Standford University. Results and conclusions: HBsAg prevalence was 10,57 percent and 10,64 percent among IDU and FSW groups in Haiphong. Three HBV genotypes/subtypes were confirmed, subtypes B4 was predominant (79.87 percent), follow by subtypes C1 (18.87 percent) and subtypes I1 (1,26 percent). Two subtypes I1 strains were indentied. Subtype I1 was identified with very rarely prevalence in Vietnam. Lamivudine resistant rate was 3,84 percent (L80I mutation: 1,92 percent
M204I mutation: 1,92 percent) and tenofovir resistant rate was 1,92 percent (M204I mutation). The resistant rate of adefovir and entercavir was 0 percent. M204I mutation (YMDD =YIDD) was cross-resistant mutation, HBV will resistant with lamivudine, tebivudine and tenofovir. The appearence of cross-resistant mutations can expect treatment failure with multi-drugs in future. The buiding a national program to monitor the appearence HBV drug resistant mutations is very importance.