Three strains of a filterable virus were isolated from the sera of children living on the outskirts of Cairo, Egypt. The 3 strains appeared to be immunologically identical and both neutralization and complement-fixation tests showed an apparent identity of these strains to West Nile virus although some slight crossing with both Japanese B and St. Louis encephalitis viruses were noted. The host range was also similar to that of West Nile virus and intracerebral inoculation in susceptible animals produced a marked encephalomyelitis. Chimpanzees developed a silent infection after intracut. inoculation, followed by the development of both neutralizing and complement-fixing antibodies. The 3 strains of Egypt virus were not neutralized by human gamma globulin collected in the U. S. A., nor by 24 different hyperimmune sera, each containing antibodies to an antigenically distinct virus type. Infection with this virus has been widespread in the local Egyptian population in 1950, with more than 70% of the inhabitants aged 4 yrs. and over having both neutralizing and complement-fixing antibodies.