An outbreak of diarrheal disease due to Yersinia enterocolitica serotype 0:5 , biotype 1, involved nine hospitalized patients. The possible source of this infection was two patients who were both hospitalized with a history of several weeks of intermittent diarrhea. Person-to-person contact was the probable mode of transmission of the disease. The present report describes the second known nosocomial outbreak of infection due to Y. enterocolitica, Serotype 0:5 of Y. enterocolitica is not commonly encountered, and its involvement in an outbreak of gastroenteritis has not been previously reported. The importance of the enrichment technique in the isolation of Y. enterocolitica is demonstrated.