Hitherto Japanese B encephalitis (JBE) virus has been isolated in many instances from men, animals and mosquitoes and it has been thought probable that JBE virus is the only prevalent arbor virus in Japan. In the meantime, Ando et al. (1952) reported the isolation of a virus (Negishi virus) from the autopsied brain of a patient clinically diagnosed as JBE. By our recent studies, evidences have been accumulated which indicate that Negishi is a member of Group B virus bearing antigen closely related with Russian Spring Summer Encephalitis virus. However, neither virus isolation nor discovery of a human infection has been reported ever since 1949. More recently, Scherer (1958) reported that he could isolate a new Group A virus (Sagiyama virus) from mosquitoes in Saitama, Japan.Taking those facts in account, it seems important to pursue the distribution of arbor viruses present in Japan over a period of years, hoping that such an investigation might give a clue to learn the ecology of arbor virus in nature. In a limited area, Gumma Prefecture, an investigation was started to gain detailed informations on the yearly appearance of arbor virus in arthropod. The present paper reveals the resuls obtained in the first year on the infection of mosquitoes in summer. The details of the isolation of 3 arbor viruses including a new one will be presented.
