The genus Yersinia contains three species pathogenic for humans. Yersinia enterocolitica and Yersinia pseudotuberculosis cause enteric diseases, while Yersinia pestis is the causative agent of plague, the most dangerous of known bacterial diseases. The phenotypic properties of Yersinia are determined by plasmid as well as chromosomal genes. In the early 1980s, several groups of researchers showed that all three Yersinia species pathogenic for humans possess plasmids. Each of the three species carried a closely related 70 to 75-kb plasmid associated with virulence (Gemski et al., 1980; Zink, et al., 1980; Ben-Gurion and Shafferman, 1981; Ferber and Brubaker, 1981; Portnoy and Falkow, 1981; Kutyrev et al., 1986). This virulence plasmid was later called pIB in Y. pseudotuberculosis, pYV in Y. enterocolitica, pCD in Y. pestis, or referred to generically as pCad, pYV, or the Lcr plasmid; and is responsible for Ca2+-dependence of Yersinia growth at 37°C (see Chapter 16 on the Yersinia virulence plasmid).