Abstract: Ultrastructural characteristics of the compatible interaction between Pseudopeziza medicaginis and alfalfa was studied using transmission electron microscopy (TEM). When P. medicaginis invaded host tissue, the hyphae directly penetrated the host cell wall and formed intracellular hyphae with growth in the cell and expansion into the adjacent cell. Hyphal penetration of host cell walls was assisted by greater vacuole pressure. Hyphae within host cells were wrapped by invagination of the host plasma membrane and mycelium and plasma membrane were always isolated and there was deposition of electron dense, dark material between host plasma membrane and cell wall. Pathogen hyphae continued to expand in the host plasma membrane region with a corresponding expansion in the host cell’s, plasma membranes which separated the host protoplast and from the mycelium. The hyphae separated from the plasma membrane to form a hyphal sheath. With an increase in the amount of infection, the region of host plasma membrane that was not penetrated was gradually degraded as P. medicaginis infected chloroplasts and other organelles. First, the mycelial sheath connected to the chloroplast and other organelle membranes, then the degradation of grana lamellae structure and degraded organelle tissues were deposited along the mycelium and around the cell wall. At a later stage of infection, the mycelium expanded both intra and extracellularly, but the hyphae in degraded cells had thick cell walls and the host cells filled with a lot of black material and crystalline particulate matter.