During the past decade, plague infections have persisted with undiminished importance in foci of the Americas, Africa, and Asia, while infections caused by the other yersiniae were recognized only during this decade as important agents of diarrheal and appendicitislike outbreaks in Europe, North America, and Japan. The dramatic rise of plague was abetted by the military conflict in Vietnam in the latter 1960s and persisted into the 1970s. During these years more Vietnamese people probably died of plague than American ser vicemen died of combat injuries. In the United States during these same years, the numbers of human cases of plague increased severalfold owing to well entrenched endemic foci in the sylvatic rodent species of the southwestern states., In the latter 1960s, microbiologists had changed the name of the plague bacillus from Pasteurella pestis to Yersinia pestis. The other two pathogenic species, Yersinia enterocolitica and Yersinia pseudotuberculosis, were known causes of mesenteric lymphadenitis and were believed to be rare. However, there originated a strong interest in these diseases in the European countries of Sweden, Finland, Belgium, and France. Yersiniae were frequently dis covered in persons with diarrhea and acute abdominal pain simulating ap pendicitis. This discovery sparked a worldwide surge of scientific interest in the genus Yersinia. Previous writings about the yersiniae have rarely considered plague and nonplague yersiniae together. Diseases caused by these bacteria, although their epidemiological patterns and clinical pictures are very different, have some striking similarities.