In June 1993 an outbreak of Shigella sonnei infection at a primary school in south east England affected 42% of 327 pupils and staff. Attack rates of diarrhoea and fever were 33% for children aged 4 to 8 years, and 8% for those aged 8 to 12 years (p < 0.00001). Illness was associated with eating canteen food (relative risk 5.9; 95% confidence interval 3.4, -10.3). All strains examined were S. sonnei phage type 3, with the same antibiogram (ttSTSS), and were indistinguishable using colicin typing and biotyping (colicin type 9, E8) and pulse field gel electrophoresis. Molecular epidemiology suggested but could not confirm that the outbreak strain was introduced into the school population from the community.