Over 55,000 small mammals of 72 species trapped in 62 administrative territories practically in all landscape-geographical zones of the USSR were examined in 1980-1985. The use of current laboratory methods demonstrated a wide spread of HFRS virus in the territory of this country, involvement in the epizootic process of most species of forest and steppe murine rodents and insectivora. In each landscape zone, the main carriers of the HFRS agent were established, represented, as a rule, by the basic species having a high and stable population density.