Summary and Conclusions: Propagation in embryonated hen egg of Ntaya, Bunyamwera, Bwamba, Uganda S, Zika, Anopheles B, Haemagogus A and B, Leucocelaenus, Sabethes and Wyeomyia viruses is reported, and the propagation in the same medium of Semliki, Mengo, West Nile, Anopheles A and Ilhéus viruses by other investigators is confirmed. With the exception of Wyeomyia, all of the viruses grew readily and could be maintained in serial passage by yolk sac inoculation using a suspension of the brain or body of the embryo for passage material. However, it was necessary to initiate cultivation of Anopheles B virus by intracerebral inoculation. The Wyeomyia virus was carried through 10 brain-to-brain passages in the embryo and was then lost. It was not infectious to embryos when inoculated by other routes. The multiplication of the viruses, their fatality to embryos and the gross lesions they produce when introduced into the yolk, allantoic and amniotic sacs and when dropped upon the chorioallantoic membrane are described. With the exception of Wyeomyia virus and initiation of the cultivation of Anopheles B virus, inoculations into the yolk and amniotic sacs proved to be the most favorable routes for consistently infecting the embryo. After 3 to 10 passages in the embryo, the majority of the viruses were found to be capable of infecting embryos when introduced into the yolk sac in dilutions as high as or higher than those required to produce a fatal infection in mice. Only 4 of the viruses (Semliki, Mengo, West Nile and Ntaya) were consistently fatal to the embryos in high dilutions. Ntaya, Bunyamwera, Bwamba, Uganda S, Anopheles A and B and Ilhéus viruses exhibited neurotropism when inoculated into the yolk sac, in that greater concentrations of the infecting agent were observed in the brain than in the body of the embryo. Semiliki, Mengo, West Nile and Zika viruses may be regarded as pantropic, as the virus concentrations in the body were equal to if not greater than those in the brain of infected embryos. Haemagogus A and B, Leucocelaenus and Sabethes viruses were consistently found in greater concentrations in the body of the embryo than in the brain. They also exhibited other characteristics in common with each other and with FA mouse encephalitis virus. The implications of these observations upon the use of the embryonated egg for the primary isolation of viruses of this category have been briefly discussed.