The cluster roots of Grevillea robusta A. Cunn. ex R. Br. are composed of determinate rootlets that stop growing, but remain physiologically active for several months. Their apical organization, both before and after maturation, was studied by light and transmission electron microscopy. Each cell layer forms a dome, with an initial cell at its end. Xylem elements form a complicated triarch array at the base of the rootlet, passing along the rootlet as two files, and then joining at the tip to form a single file, surrounded by six pericycle cells. At the base of the rootlet, shorter xylem cells and thick-walled support cells are visible. A root cap, present in rootlets grown in vermiculite, was eventually displaced by root hair growth. Rootlets grown in Hoagland's solution lacked root caps and were significantly shorter than those grown in vermiculite. Cell fate was analysed in terms of cell position and is discussed in terms of pattern and development.