BACKGROUND Sentinel surveillance of the antimicrobial resistance of strains isolated from female sex workers in North Sumatra, Indonesia, has been carried out since 1975. In 1996 a high prevalence of strains with plasmid-mediated resistance to tetracycline and penicillin was observed. OBJECTIVE The goal was to further characterize strains isolated from a core group of patients in Indonesia with sexually transmitted infections in 1996. METHODS The strains were characterized by antimicrobial susceptibility testing, plasmid analysis, subtype of the determinant, and analysis of genomic DNA by pulsed-field gel electrophoresis (PFGE). RESULTS A total 161 strains obtained from 592 female sex workers in 10 different places in North Sumatra, Indonesia, in 1996 were investigated. All strains exhibited plasmid-mediated resistance to penicillin (PPNG: penicillinase-producing ) and/or tetracycline (TRNG: tetracycline-resistant ); 115 strains were PPNG/TRNG (71%), 45 were TRNG (28%), and 1 was PPNG. All strains were susceptible to ceftriaxone, ciprofloxacin, kanamycin, and spectinomycin. All PPNG strains tested carried the 7.2-kb (Asian type) plasmid except one, which carried the 4.9-kb (Toronto type) plasmid. All TRNG strains except one contained the Dutch-type gene. PFGE analysis of 156 strains documented that a diversity of strains existed and that certain genotypes had spread in a defined area or between different areas in North Sumatra. CONCLUSIONS Our results underline the importance of continuous surveillance of the changing patterns of antimicrobial resistance of in high-risk populations.