1. Western equine virus has been successfully transmitted in the laboratory by 3 species of mosquitoes from 2 genera not previously reported as laboratory vectors: Culex tarsalis, Culiseta inornata, and Culiseta incidens.
2. Though transmission was not demonstrated, survival of the virus for more than a few days was shown to occur in Culex stigmatosoma and Psorophora confinnis. Possibly transmission occurred by the former.
3. In experiments with Culex tarsalis, infection of the mosquitoes occurred from feeding on an inoculated guinea pig, a duck, and virus-blood suspensions. After an incubation period of 10 to 30 days at a temperature above 25°C. these mosquitoes infected chickens and a guinea pig by their bite and virus was in turn demonstrated in the blood of the chickens and in the brain of the guinea pig. A total of 12 transmissions occurred. The fact that mosquitoes can be infected from fowl and in turn transmit to fowl, together with much other supporting data from field and laboratory, is interpreted as strengthening evidence that fowl serve as reservoirs of virus in nature.
4. Since Culex tarsalis mosquitoes have been repeatedly found infected with Western equine virus and epidemiologic evidence supports their incrimination, the vector rôle of this species is now established, and it may be regarded as fully incriminated.
5. Culiseta inornata has also been found infected in nature and now proven a laboratory vector. This species does not fit the epidemiological picture in the Yakima Valley as well as C. tarsalis, but may play an important rôle elsewhere.
6. Anopheles maculipennis freeborni and Culex pipiens found naturally infected have not transmitted the virus under laboratory conditions.