Since 1983 the incidence of resistance to ampicillin in Shigella dysenteriae, Sh. flexneri, and Sh. boydii infections in England and Wales has increased from 42% to 65% and the incidence of resistance to trimethoprim, from 6% to 64%. Furthermore, of 1524 strains received in 1995-1996, 46% were resistant to both of these antimicrobials. For Sh. sonnei almost 50% of isolates were resistant to ampicillin or trimethoprim and 15% were resistant to both of these antimicrobials. These results demonstrate that if antibiotic therapy had been indicated for infections with Sh. dysenteriae, Sh. flexneri, and Sh. boydii, then treatment with either ampicillin or trimethoprim may have been ineffective in almost 50% of cases and for Sh. sonnei, in 15% of cases. It is concluded that if it is necessary to commence treatment before the results of laboratory-based sensitivity tests are available, the best options would be to use nalidixic acid for children and a fluoroquinolone antibiotic such as ciprofloxacin or ofloxacin, for adults.