Leek (Allium porrum L. cv. Titan) roots wore colonized by the vesicular arbuscular (VA) mycorrhizal fungus Glomus versiforme (Daniels and Trappe) Berch. External hyphae were of varying sizes and had a single-layered wall surrounding a vacuolate cytoplasm. Intra-hyphal hyphae were frequent in the extraradical mycelium. Some external hyphae formed small branches which initiated simple appressoria over the outer tangential wall of epidermal cells. Appressoria were multinucleate and possessed many structures which we have interpreted to be bacterium-like organclles (BLOs). Infection hyphae formed by appressoria penetrated the radial cell walls of epidermal cells and were surrounded by an electron dense substance and host cell wall. Epidermal and cortical cells reacted frequently by forming cell wall thickenings. Hyphae in the exodermis formed coils which were surrounded by host plasma membrane but not host cell wall, and had many nuclei and BLOs.