We report the clinical, microbiological, and epidemiological features of an emerging serotype, Shigella boydii 20. We interviewed patients about symptoms, and history of travel and visitors during the week before illness onset. Seventy-five per cent of the 56 patients were Hispanic. During the week before illness onset, 18 (32%) travelled abroad; 17 (94%) had visited Mexico. Eight (21%) out of 38 who had not travelled had foreign visitors. There were eight closely related patterns by PFGE with XbaI. S. boydii 20 may be related to travel to Mexico and Hispanic ethnicity. Prompt epidemiological investigation of clusters of S. boydii 20 infection may help identify specific vehicles and risk factors for infection.