Three strains of Newcastle disease virus (NDV-HP-16, NDV-L-Kansas, and NDV-N) were propagated in chick embryo fibroblasts, equilibrium labeled with 32Pi, and the composition of phospholipid in the membranous envelope of the virions determined. A phospholipid identifed as monoacylphosphatidylserine was consistently observed in the viral strains which are listed as follows in their order of decreasing abundance of lysophosphatidylserine: NDV-HP 16greater than NDV-L-Kansas greater than NDV-N. The phosphatidylserine concentration in the virion envelopes of these strains decreased in proportion to the increase in the monoacylphosphatidylserine concentration. No other lysophosphatide was observed in significant quantity in virions of these strains. The degree of cell fusion in mouse fibroblast monolayers by each of the viral strains was independent of the lysophosphatidylserine content of the virions. The ability of the viral strains to induce fusion from within, i.e., that occurring in cells that are actively propagating virus was: NDV-L-Kansas greater than NDV-HP-16 greater than NDV-N. The ability of the viral strains to induce fusion from without, i.e., that occurring in response to incubation of cells with large quantities of irradiated virus was: NDV-HP-16 greater than NDV-N greater than NDV-L-Kansas. On the basis of these findings we conclude that there is no direct correlation between the level of lysophosphatide in the virion and its ability to induce cell membrane fusion. A direct correlation was observed, however, between the presence of high monoacylphosphatidylserine content and the ability of a strain to produce lytic infection.