The paper deals with one of the most important problems of plague natural foci, namely the places and types of its bacillus persistence outside the parasitic system, as well as in the interepizootic period. It also discusses different hypotheses forwarded on the basis of field observations and experimental findings. In particular, it gives the materials suggesting that the plague bacillus can persist in the low-virulent form under natural conditions, as well as due to its transition to a L-form. The paper provides strong evidence for the long-term persistence of the plague bacillus in soil and hole substrates and discloses the adaptive mechanisms ensuring such persistence. Particular emphasis is placed on the complex symbiosis of the plague bacillus with soil microorganisms, including protozoa, and on their persistence in cysts of the latters. Tasks and a trend in further studies required to finally solve the problem are stated in the paper.