Mosquitoes (eggs, larvae, and adults), small woodland animals, and residents of an area where California encephalitis is endemic were studied to elucidate the host-vector cycle of La Crosse virus. Elementary schoolchildren from surrounding communities and gray squirrels from another area were tested to compare the prevalence of serum antibodies to La Crosse virus in areas where the disease is endemic with the prevalence of these antibodies in areas where the disease is not endemic. From 1971 to 1974, eight isolations of La Crosse virus were made. Three of the isolates were from adult mosquitoes, one from Aedes triseriatus eggs, and four from A. triseriatus larvae. The isolation of virus from field-collected eggs and larvae confirms previous studies from Wisconsin that suggest that La Crosse virus overwinters in eggs of the mosquito A. triseriatus. In an area where California encephalitis is endemic, 10 of 19 small woodland animals (53%), which are the natural hosts of A. triseriatus, had hemagglutination-inhibiting and neutralizing antibodies to La Crosse virus. In contrast, none of 10 squirrels from an area where the disease is not endemic had such antibodies. Fourteen of 79 residents of this area (17.7%) had both types of antibody. Eleven of the 14 seropositive residents lived in one small sector of the community studied, an indication that foci of La Crosse virus activity may be very localized. Elementary schoolchildren from surrounding communities had a significantly lower prevalence of hemagglutination-inhibiting antibody to La Crosse virus than did the residents of the area where California encephalitis was endemic.