An epidemiologic investigation was carried out to identify the source of infection of typhoid fever outbreak, which was occurred in Nam-Gu, Pusan city from April 20 to June 28, 1996. The distribution of 83 reported cases (55 confirmed cases and 28 suspected cases) by sex and age group showed that this epidemic would be occurred by the exposure to a common source with low toxic pathogen because 55.4% of cases were under 20 years old and most of the adults cases were female. The epidemic curve showed a unimodel curve with a peak on about 15 May and tailed down to 23 June, suggesting that the exposure was constant. These results supposed that the contamination of a public water supply would be the source of infection. The analysis of reported cases and population survey conducted to household members and community contacts living in the epidemic area, led the conclusion that the source of infection was tap water supplied through the Dae Youn pumping station, based on the following findings: (1) There were singificant differences of incidence rate among regions (Tong) even though in the same Dong and the spot map of incident cases distributed in accord with the water supply by Dae-Youn pumping station. (2) Among reported cases, 21 cases, including 17 confirmed cases, stated they had used only tap water. (3) The analysis of tap water from the house which had seven typhoid cases, revealed the contamination of E coli and the dilution of residual chlorinated concentration. (4) The outbreak of typhoid fever was stopped after the replacement of The worn-out pipe line of tap water supply to the new one.