We have experienced an outbreak of dysentery in Nagasaki. Shigella sonnei were positively cultured from 467 patients out of suspected 821 cases, and 346 patients were admitted. 121 patients were treated with oral antimicrobials in the outpatient clinic. Five patients were diagnosed as secondary infection. We treated a total of 96 patients in Nagasaki Municipal Medical Center, and studied the clinical and bacterial features in these 96 patients. Chief complaints included fever, abdominal pain and diarrhea. Most diarrheal patients showed waterly diarrhea and only a few were bloody (3 of 47). Treatment of levofloxacine 300 mg a day for 5 days successfully eliminated S. sonnei from all culture positive patients. An environmental surveillance revealed that water in a well at the university to which many patients were using was the origin of the infection with positive cultures of S. sonnei. No difference between the clinical and environmental isolates was observed in results on biochemical, serological and enzymatic tests. All isolates were susceptible to levofloxacin and to ofloxacin, but three isolates showed resistance of fosfomycin with MIC above 64 micrograms/ml. In analysis of pulsed-field gel electrophoresis, both clinical and environmental isolates were considered to be closely related.