Shigella sonnei is a significant cause of diarrhoeal infection in both developing and industrialized countries. From 1999 to 2003, 445 strains of Shigella sonnei were isolated from patients admitted to the diarrhoea treatment centre of the International Center for Diarrhoeal Disease Research, Bangladesh. More than 60% of the isolates were resistant to nalidixic acid, 89% to sulfamethoxazole-trimethoprim and 9.5% to ampicillin. In addition, 4% of strains were resistant to multiple antibiotics (AmpR TetR SxtR StrR) and 4.2% of strains were sensitive to all antibiotics tested. None of the strains were positive for the set1 gene, whereas 46% were positive for the sen gene. Forty-six per cent of the strains (stored at -70 degrees C) harboured the 120 MDa invasive plasmid and representative strains produced keratoconjunctivitis in the guinea pig eye. In addition, three plasmids of approximately 5, 1.8 and 1.4 MDa were found to be present in more than 90% of the strains. A self-transmissible, middle-ranged plasmid (35-80 MDa) carrying the multiple antibiotic resistance gene was found in some strains. PFGE analysis of the strains identified five unique types with many subtypes, which were characterized into four unique types by ribotyping analysis. It can be concluded that endemic strains of Shigella sonnei isolated from patients in Bangladesh are diverse in their genetic pattern.