The paper characterizes the seasonal phenomena of the life cycle of fleas in relation to the intensity of epizootias in the natural foci of plague in the Caucasus. A situation determined by the vital activity of several species of fleas, the parasites of a major vehicle, is, in terms of pathogen transmission, established in each of 9 natural plague foci. They are combined by the nesting-borrow type of parasitism, which provides the wide distribution of plague pathogen among rodents. In each focus, there are the most intensive epizootias in the period of high feeding and reproductive activity of transmitters.