The ultrastructure of Olpidium brassicae was observed in epidermal cells of lettuce roots from 8 h after inoculation with zoospores until the thalli matured, at 72 h. In early stages of growth the thallus is completely surrounded by the host cytoplasm and separated from it by the thallus ectoplast. A thallus wall begins to form after 36 h and consists of three layers. The thallus contains the usual organelles, a nucleus (many nuclei after about 24 h), mitochondria, ribosomes, and lipid globules. Membrane-bounded structures, the multivesicular bodies, occur throughout the life cycle and are thought to be involved in synthesis of glycogen and lipid, and at a later stage in cleavage of the thallus protoplasm into zoospore protoplasts. One kinetosome in each zoospore protoplast produces a flagellum that is pushed into the cleavage vacuole. An exit tube originates from a bulge in the thallus wall and is closed by an elaborate plug.