The mycorrhiza of Ophioglossum reticulatum was studied by means of light microscopy, scanning and transmission electron microscopy. An aseptate multinucleate fungus forms intracellular hyphal coils in root cortical cells. Single hyphae from these coils penetrate adjacent host cells and form arbuscules. In the youngest infected host cells arbuscules are of a coralloid shape, but on older host tissue, they develop large, terminal vesicular swellings. The arbuscules are surrounded by matrix material and the host plasmalemma. Host cytoplasm and plastids increase with arbuscule development and starch disappears. Living and degenerated arbuscules may occur within the same host cell. Intercalary hyphal swellings and single terminal vesicles are found in the older host tissue where arbuscules have degenerated.