Summary Nodamura virus, an ether- and chloroformresistant arbovirus, uniformly passed filters with pore sizes of 100 but not 50 mµ, and 55 to 75 mµ virus-like particles were seen in infected mouse brain and heart. It produced limb paralysis, inflammatory and degenerative lesions in centralnervous-system tissues, skeletal muscle and brown fat, and death of suckling mice 5 to 15 days after intracranial, subcutaneous, or intraperitoneal inoculation. Suspensions of mouse limb muscle contained higher concentrations of infectious virus than brain suspensions. Antibody formation without disease occurred after one sc inoculation of a pig and after three or four ip or intramuscular inoculations of adult mice and rabbits; however, neutralizing antibody did not form in baby chicks after one sc inoculation. No overt illness occurred after inoculation of baby chicks (ic or im), weanling mice and rabbits (ic), weanling guinea pigs and Syrian hamsters (ic, ip, and im), or embryonated chicken eggs (chorioallantoic membrane, amniotic sac, or yolk sac). No cytopathologic effects were seen within 9 to 25 days after inoculation of primary cell cultures of Syrian hamster, porcine, calf, rabbit, suckling Swiss albino mouse and cynomolgus-monkey kidney, whole chicken embryo and human amnion, or of continuous cell cultures from human cervical carcinoma (HeLa), human liver and conjunctiva (Chang), C2H mouse subcutaneous tissue (L), rabbit papilloma, and rabbit skin. Infection of swine was common on farms near Tokyo during 1956–57, but only one of 54 herons and egrets, none of 29 rural residents, and none of 13 urban residents near Tokyo showed N antibody in plasma collected during 1956–57.