An outbreak of water-borne infection caused with Yersinia pseudotuberculosis serovar O3 occured in Anan town, Nagano prefecture during May to June 1988. A total of 21 elementary school pupils. 10 junior high school pupils and 3 high school students were affected. There was no death, nor secondary infection. Positive rate of isolation of Y. pseudotuberculosis from the feces of patients and healthy persons was 41.2% and 9.5%, respectively. In the outbreak, Y. pseudotuberculosis was the only suspected etiological agent isolated from patients. Sera from 25 patients (73.5%) showed elevated agglutinin liters, ranging from 1 : 80 to 1 : 5,120, against the somatic antigen of the isolates. Sixteen strains isolated from patients and healthy persons possessed approximately 42 Md plasmid and the results of calcium dependency, pyrazinamidase activity and autoagglutination were positive. This was the first report on the outbreak of infection due to this organism in Nagano prefecture.