The authors present published data and their own findings on the relationship between Yersinia pestis and Y. pseudotuberculosis and on the origination of Y. pestis from Y. pseudotuberculosis. Study of microbiological and biochemical characteristics, external membrane protein spectra, and stability of chromosomal region of pigmentation brought the authors to a hypothesis that Y. pestis minor subspecies (ssp. caucasica, altaica, hissarica, ulegeica) which are characterized by selective virulence occupy an intermediate position between Y. pseudotuberculosis and basic species of Y. pestis.