Data characterizing the reservation forms of plague infectious agent in Tuva natural focus are presented in the review. Yersinia pestis was shown to persist most of the year in Citellophilus tesquorum altaicus imago --the main carrier, getting into the animal organism only for a short time. An increased ability to aggregate in autumn and accumulate in clumps of C. tesquorum altaicus females that are more adapted to survive the cold season compared with males promote the persistence of the microorganism. The plague infectious agent in the altered form survives in the organism of females not only the winter period but also longer periods of time that is demonstrated by the facts of detection of it after 646 days of staying in the carrier. Moreover Yersinia pestis can persist for more than 400 days in the substrate of the nest of long-tailed ground squirrel infected by excrements and corpses of plague fleas. The substrate of the nest infected in summer-autumn period of the previous year may determine the primary infection of ground squirrels by plague infectious agent in the next epizootic season. On ground squirrels infected during contact with nest substrate, infection of intact fleas may be possible, and so the initiation of a new cycle of transmission of the infectious agent. Adaptation of the plague infectious agent to unfavorable existence conditions in the carrier is expressed in the changes of its morphology and ultrastructure that is evidenced by the facts of isolation of the infectious agent from corpses of fleas situated in the substrate, in the L-form, as well as results of phase-contrast and electron microscopy of the digestive tract of C. tesquorum altaicus.