Eight isolates of La Crosse virus were obtained from 182,437 mosquitoes (4,643 pools) processed during 4 years of a 5-year study. Of the 8 isolates, 5 were from 1,985 Aedes triseriatus (70 pools), and 1 each was from 1,818 Culex pipiens, 35,867 Aedes communis group, and 36,800 Aedes trivittatus. Six of the isolates were from mosquitoes collected by aspiration, and the other two from Culex pipiens and Aedes trivittatus collected in carbon dioxide-baited cone traps. Seasonal distribution of the isolates was similar to that of cases of California encephalitis: 3 in July, 2 in August, and 3 in September. Aedes triseriatus is common in hardwood deciduous forests where in small wild mammals antibodies to the virus are prevalent and where children hospitalized with the disease have been exposed to the virus in nature.