The growth of Newcastle disease virus in 12 day old embryonated eggs was studied. Data are presented to support the contention that there is a periodicity in the growth of the virus. The max. adsorption of the virus inoculum onto susceptible cells of the chorio-allantoic membrane occurred within 10 min. of its introduction. Employing irradiated homologous virus it was possible to produce an interference phenomenon and demonstrate that the virus proliferated in cycles of 4 hrs. The existence of an unequal development of infective and hemagglutinating titers during the growth of New castle disease virus in the fertile egg is described. It was possible to demonstrate infectivity titers early in the growth period while the hemagglutinating property was absent. Hemagglutination titers appeared later and continued to rise after the infectivity titers became stationary. This discrepency can be explained on the hypothesis that the hemagglutinating property of the virus results from a maturation of the infectious particle and, therefore, cannot be detected early in the growth period.