Attendees of an outdoor music festival that was held in Michigan became ill with Shigella sonnei. The onset of the illnesses peaked two days after the festival ended, after attendees had returned home. An uncooked tofu salad served on the last day was associated with risk of illness. A smaller outbreak of Shigella sonnei occurred among festival volunteers before, and during, the festival. There was limited access to soap and running water for handwashing.