The transmission efficiency of LaCrosse encephalitis virus (California group) by 9 species of mosquitoes was tested in the laboratory in comparison to the efficiency of Aedes triseriatus, a known biological vector in nature. Transmission rates of 70–77% to suckling mice were observed when A. triseriatus ingested a virus-blood suspension containing virus concentration of 104.4 SMLD50/0.03 ml of blood. At similar dosage levels, Aedes vexans, Aedes canadeiisis, Aedes aegypti and Culiscta inoniata were capable of transmission but at much lower rates, i.e., 27%, 25–27%, 10%, and 33%, respectively. Aedes stimulans and Anopheles punctipennis failed to transmit virus and Aedes trivittatus showed low transmission rates dosages of 105.6 and 105.8 SMLD50/0.03 ml. Virus titers of individual A. vexans, A. canadensis and A. trivittatus indicated poor virus multiplication in these species. The tests indicated low potential for all of the species except A. triseriatus as biological vectors of the virus in nature.