Arthropod-borne animal viruses (arbor viruses) are known to be widely distributed throughout the world. In Japan, Japanese B encephalitis (JBE) virus has been most frequently isolated from mosquitoes as well as from human infected cases. Dengue type I virus was isolated out of the human blood during epidemics which occurred in the World War II (Sabin, 1952) . In the summer of 1948, 2 strains of virus were isolated from encephalitic patients and one of those, Negishi strain, was recently identified as a member of Russian spring summer encephalitis (RSSE) subgroup arbor virus (Ando et al., 1952; Okuno et al., 1961) . More recently, a group A virus, Sagiyama, was reported to have been isolated from mosquitoes caught at Kanto plain (Scherer, 1958) .
Since 1959, a long term project to investigate the ecology of arbor viruses in Japan was set out by our hand and interesting findings are having been accumulated including the isolation of hitherto undescribed arbor virus (Matsuyama et al., 1964) . The present report reveals the biological and immunological properties of this new virus.