Two distinguishable subpopulations were recovered from chickens infected with lentogenic strains of Newcastle disease virus originally ioslated from chickens in Australia and Ireland. In both instances, a subpopulation that rapidly eluted from chicken erythrocytes was recovered predominantly from tracheal swabs, and a slow-eluting subpopulation from cloacal swabs. The two subpopulations differed in thermostability of the hemagglutinatin and mean survival time of infected chicken embryos, as well as in elution pattern. The heterogeneity characteristic of the Aust-V4 strain and NI-Ulster strain could not be demonstrated in B1, NJ-LaSota, and ENG-F, lentogenic strains commonly used in vaccines. The question is discussed of whether standard vaccine strains are also hetrogeneous if a technique for separating stock populations is found, and the significance of possible heterogeneity to the effectivenss of the vaccines.