A study in Bangkok in 1988 found resistance by the four Shigella species to commonly used antibiotics (11). Overall, 87% of Shigella isolates were resistant to ampicillin, 84% to co-trimoxazole, and 0.1% to nalidixic acid. We found that by 2003, at least 90% of S. flexneri isolates were resistant to ampicillin, co-trimoxazole and tetracycline, probably reflecting a survival advantage for resistant strains in the presence of frequent consumption of antimicrobials in the study site.