The enteropathogenicity of Providencia alcalifaciens, a member of the family Enterobacteriaceae, has not yet been well established. In November 1996, a large outbreak of foodborne infection occurred in Fukui, Japan. In this study, the etiology of the outbreak was investigated. No other recognized enteropathogens were detected in patient fecal samples, but P. alcalifaciens was detected in 7 of 18 samples. The isolates were found to be clonal by pulsed-field gel electrophoresis. The patients who presented with gastroenteritis had elevated levels of specific antibody against the isolated P. alcalifaciens. The isolates showed invasion of Caco-2 cells and fluid accumulation in rabbit ileal loops. This study strongly suggests that the outbreak was caused by P. alcalifaciens. This is the first report of a large outbreak of foodborne infection attributed to the organism and provides definitive evidence that P. alcalifaciens is a causative agent of gastroenteritis.