The majority of Shigella flexneri strains isolated in England and Wales are from infections contracted abroad. Most of these strains are drug-resistant, over 75% being resistant to streptomycin and sulphonamides, sulphonamides alone or streptomycin, sulphonamides and tetracyclines. A selection of resistant strains was tested for resistance transfer and the plasmids identified were characterized by compatibility grouping. Streptomycin and sulphonamide resistance was usually determined by a non-autotransferring plasmid which may be mobilized by standard transfer factors, or by the plasmid which conferred tetracycline resistance where this was present. The remaining resistant strains were predominantly resistant to four or more drugs. These strains carried autotransferring plasmids of a variety of compatibility groups, of which groups B, I1 and FII were the most common.