Abstract: Abstract: Aim of this study was to determine the causative agent and source of a large gastroenteritis outbreak occurred in a national financial center (CBIRI) in July 2003. A patient definition was defined staff were interviewed in the clinic of the Bank and their information were collected by means of a standardized questionnaire. A total of 110 fecal specimens were collected within 48 h of symptom onset from 100 patients with symptoms of gastroenteritis and 10 restaurant staff. The specimens were processed within 12 h to detect ova and parasites by direct microscopy and common bacteria by standard methods. The outbreak started on 22 July 2003 lasted 4 days. From a total of 1300 staff. 535 persons experienced a severe gastrointestinal illness. None but one of tested fecal samples were positive for bacterial enteric pathogens. S. paratyphi B was isolated from the positive case. Definitive association between illness and isolated S. paratyphi B remained to be determined since it was isolated only from one case. There is a need, however, for increased awareness among both professionals and the public to implement appropriate prevention measures and monitoring of food and water.