Russian spring-summer encephalitis (RSSE), known as forest encephalitis or tick borne encephalitis, is disease Smorod.intseff and his co-workers (1) have made clear of its epidemiological aspects, causative agent, mode of infection and preventive measures during the years 1937-39. The disease prevails mainly in the Far-Eastern Russian terriory and is also reported since 1932 in European Russia and in Siberian woodlands. Kitano (2) has reported of the disease breaking out on the Manchuria-Soviet Frontiers and was successful to isolate the causative agent. Kitaoka (3) also mentioned that the disease was reported among those Japanese war prisoners engaging in wood-cutting at Sosoevka woods beyond the highlands in the north of Vladivostock in the II World War and the disease was found breaking out during the course of April 19 June 20, and about 10-16% of the total workers have contracted it, resulting in 19-50% casemortality. Symptoms, after invasion of the virus, and as far as to the studies on the neutralizing antibody in the convalescent serum are discussed about in, the report too.The virus is spread by Ixodes persulcatus Schuize, a species of wood tick and it was also made clear by Pavlovsky (4) that Eutamia asiatica, a rodent, is known as reservoir.Casals (5) and Hammon (6) have described that causal agent is not single and there are two types, the Far Eastern and the Western types, which Kitaoka (3) has also approved by the neutralizing test with the convalescent serum from the RSSE patient and has made clear that the former type is peculiar in the nervous disturbances and extrapyramidal symptoms, while the latter chiefly presents symptoms from irritations on meninges (Yamazaki) .Smorod intseff (1) has mentioned of the disease being not spread by mosquitoes. But it is of interest to see if it may be transmitted by mosquitoes because the vector of the encephalitis occurring in summer season is, as Hammon (6) and Kitaoka (7) have mentioned, mainly a mosquito. The RSSE prevails most vigorously during late. spring towards the summer when mosquitoes grow thrivingly. Well an experimental study on transmission by mosquitoes of the RSSE viruses namely, the Sophy strain (Far Eastern type) and a Western type, which were given from Dr. Hammon was carried out.