Experiments were carried out to study the mechanism of which Potato X Virus (PXV) is not transmitted by aphids. Green peach aphids (Myzus persicae SULZER) transmitted only Potato Y Virus (PYV) to the healthy tobacco (Nicotiana tabacum var. Xanthi) plants from Potato crinkle mosaic (PXV+PYV) diseased tobacco, and did not transmit PXV to the tobacco plants. NISHI (1958) have demonstrated the inhibitory effect of substance secreted by aphids on feeding plants upon the infectivity of Tobacco mosaic Virus (TMV). In the present experiments, pressed juice of tobacoo plants affected by PXV was mixed with alcohol precipitated substances in pressed sap from chinese cabbage (Brassica pekinensis RUPR.) on which green peach aphids had been fed. The virus juice was tested for infectivity by local lesion method using Gomphrena globosa. The substances secreted by aphids on feeding plants inhibited the infectivity of PXV strongly as in the case of TMV.