Prolonged study of the population of Arvicola terrestris L. in connection with tularemia epizootic among these animals showed this animal to serve as the principal carrier of tularemia infection in the Povorinsk natural focus. Epizootics proved to originate with a definite threshold count of Arvicola terrestris L.; this threshold constituted about 30% of the trapped animals. Dynamics of the epizootic activity was characterized by alterations of cycles of various duration, coursing by waves, with 2--3-year periods. Reduction of the epizootic intensity coursed during a decade in 2 waves with 3-year periods, and during a 7-year period--in one 3-year wave. During the epizootics with the character of a 2-year wave the population of the principal carrier diminished, and with a 3-year wave the count of Arvicola terrestris L. decreased during the first year, and began to increase during the second year, despite the persistence of the epizootic. Detection of regularities attending the changes in the population of the principal carrier of the infection and its connection with the epizootics permitted to make long-period prognoses of the epizootic activity and to plan antiepidemic measures in the focus.