[en] Newcastle disease virus (NDV) propagated in monolayers of chick embryo fibroblasts (M-NDV) has been found to differ morphologically from NDV propagated in embryonated eggs (E-NDV). Virions of M-NDV sediment faster, are lower in density, are about four times larger, and are more fragile to degradative forces encountered during high speed pelleting and rebanding in CsCl than E-NDV virions. These differences have been shown to be host controlled and are not due to contaminant virus, differences in growth medium, virion clumping, or time of harvest. Although both E- and M-NDV populations contain multiploid virions, these virions do not exhibit multihit inactivation kinetics. The possible genetic consequences of the differences between E-NDV and M-NDV are discussed