Observations at ultrastructural level have been added to earlier descriptions of the vascular tissue inParthenium argentatum (guayule). Large transfer cells with pronounced cell wall projections were found in the phloem of the minor veins in the leaves. These cells have every appearance of being modified for the purpose of collecting photosynthates. In the large veins of the leaf the elements of the phloem, engaged in bulk transport of solutes, showed reduced transfer cell characteristics. Within these larger veins of the leaf nacreous thickenings of the sieve tubes was very common. Similar thickenings were found in the petiole and the stem and these persisted to maturity. Histochemical tests indicated a high level of pectic substances in these walls. In the stem, particularly in regions where rubber deposition was significant, the companion cells in the phloem had elaborate cell wall ingrowths. The adjacent parenchyma cells were adequately provided with pits. Occasionally transfer cells were found in the xylem.