Twenty-nine isolations of California encephalitis complex (CAL) viruses were obtained when suckling mice were inoculated with 3,609 pools of 123,464 mosquitoes collected in northeastern New York State during the summers of 1976 and 1977. Vero cell cultures subsequently inoculated with 1,092 of the negative pools yielded 2 additional isolates. These finding, together with results of parallel titrations of 8 virus-positive mosquito suspensions in both systems, indicated that the detection of CAL viruses in wild-caught mosquitoes was optimal when Vero cell cultures were used as a supplementary isolation system to suckling mice.