Microfilaments 50–70 Å (Å = 0.1 nm) in diameter are commonly found in vascular tissue of elongating roots and stems of many plants. Such microfilaments occur in bundles in peripheral regions of elongating or differentiating vascular cells, and are usually oriented parallel to the longitudinal axis of the cells. Although the longest microfilament bundle we have measured is about 15 μm, we suspect that the bundles are at least as long as the cells that contain them. The bundles are 0.1–0.4 μm in width and are often in apparent contact with organelles, but there is no definite indication of their being anchored to plasmalemma or any organelle. The microfilaments are comparable in size and morphology to actin filaments. The distribution of microfilament bundles in the vascular tissue of various plants and the possible function of the bundles is discussed.