Seroepidemiological studies revealed the existence of a population stratum immune to West Nile and Sindbis viruses in the Saratov region. Screening of 218 sera showed that the stratum immune to arboviruses increased from 1998 to 2000. The incidence of antibodies to Batai and Sindbis viruses was the highest. Activation of natural foci of these arboviruses can lead to exacerbation of the epidemic situation in the region.