Plaque reduction-serum dilution neutralization was used to evaluate the status of bunyavirus activity in deer in mountainous areas of California. Antibodies against 9 bunyaviruses were measured in 337 mule deer (Odocoileus hemionus hemionus, O. hemionus californicus, and O. hemionus inyoensis) and black-tailed deer (O. hemionus columbianus). More deer from high mountainous areas had neutralizing antibodies against Jamestown Canyon virus than did deer from low mountainous areas (23% vs. 9%; P less than 0.01). This finding is consistent with transmission by snow pool Aedes mosquitoes. Results for Jerry Slough virus were nearly identical to those for Jamestown Canyon virus, which is further evidence that these are strains of the same virus. Neutralizing antibodies against Northway virus were present in 26% of deer from high mountainous areas and 23% of deer from low mountainous areas, suggesting the involvement of a widespread vector, such as Culiseta inornata. Northway virus is not known to occur outside of Alaska and northwestern Canada. Low prevalences of antibodies were detected in deer to California encephalitis, La Crosse, and snowshoe hare viruses of the California serogroup; and Cache Valley, Lokern, and Main Drain viruses of the Bunyamwera serogroup.