OBJECTIVES: This study was carried out to investigate the sources of infection and modes of transmission of an outbreak of shigellosis that occurred among pupils of "M" primary school and residents near the school in Kyongju from Sept. 24 to Oct. 24, 1998. METHODS: The subjects who completed a questionnaire and a rectal swab for microbiologic examinations were 1,534 persons (781 males, 753 females), including 469 pupils of "M" primary school (268 males, 201 females). Bacteriological examinations of underground water and simple piped water were done. RESULTS: The attack rate of diarrhea was 28.7% in the subjects from Sept. 24 to Oct. 24, 1998. There was no difference in attack rate of diarrhea by gender, but it was significantly higher in the pupils of "M" primary school than others (p<0.01). The attack rate of diarrhea by resident areas was no different to the pupils of "M" primary school, but was significantly higher in the residents of Mohwa 2 Ri except pupils that "M" primary school is located in (p<0.01). The distribution of date of onset revealed the exposure date to be Sept, 22 and 23 in consideration of incubation periods and common source outbreak followed propagative spread in the epidemic curve. The major characteristics of diarrhea were watery (89.1%) in nature, 1~3 days (72.5%) in duration, 2~3 times (63.9%) in frequency. The clinical symptoms among the diarrheal cases included abdominal pain (74.1%), fever (56.4%), headache (55.9%), chill (40.4%) and tenesmus (31.4%). CONCLUSIONS: The source of infection was estimated to be contaminated underground water and simple piped water caused by leakage from the cess pool. It is highly necessary that the management of drinking water and cess pools should be done thoroughly.