Seventy strains of California group viruses were isolated in the Central and Southern territories of the Russian plain situated in the Southern taiga, mixed forest, broad-leaved forest, forest-steppe, steppe, and semiarid zones. Sixty-three of these were isolated from 873,300 mosquitoes, 4 from patients, and 3 from rodents. 57.1% of strains were isolated from Aedes communis and the associated species, 28.5% from Aedes excrucians and associated species, and rarely from other mosquito species. The mosquito infection rate was the highest in the Southern taiga zone: 0.0103%, this value decreasing towards the Southern areas. Out of 70 isolated strains 42.3% were identified as the Inkoo virus, 47.2% wer ejust referred to the California group, and only 2.8% were Tahyna and snowshoe hare viruses. The findings of serological screening of the population correlate with the results of virological studies of mosquitoes. Population morbidity is mainly caused by the Inkoo virus in the Southern taiga and mixed and broad-leaved forest, by Inkoo and Tahyna viruses in the steppe and forest-steppe zones, and by Tahyna virus in semiarid zone.