Yersinia pseudotuberculosis infections in two bacteriologically confirmed cases are described. A child was found to have mesenteric adenitis and an adult had septicemia. Invariably simulating acute appendicitis, mesenteric adenitis most often occurs in male children and adolescents. Septicemia with this organism usually affects elderly, debilitated patients, who frequently have chronic hepatic disease. The infrequent diagnosis of infection with Yersinia pseudotuberculosis in the United States is probably due to failure to consider it a human pathogen. Currently classified with the Enterobacteriaceae, Yersinia pseudotuberculosis in a non-lactose-fermenting, Gram-negative coccobacillus. It is sensitive to a wide range of antibiotics, including tetracycline and streptomycin, but usually is resistant to ampicillin. Yersinia pseudotuberculosis has a worldwide distribution in wild and domestic mammals and birds. Infections in man may result from direct contact with infected animals or their excreta.