An ultrastructural study was made of the vesicular-arbuscular mycorrhiza formed by Endogone (Glomus) mosseae with roots of Allium cepa. The fungal arbuscules consist of highly branched hyphae that have a characteristic reticulate vacuolation. The host plasmalemma surrounds the individual branches of the arbuscule. Around the penetration point and the trunk of the arbuscule a collar of wall material lies between the host plasmalemma and the fungal wall. This layer is absent from the branches. When the arbuscule senesces, the branches collapse and the empty fungal walls aggregate into a clump. Within the clump the fungal walls appear to fragment and disintegrate. The host plasmalemma at this stage surrounds the clump as a whole. When an entire infection becomes senescent, a thick layer of wall material apparently derived from the host encases all intracellular fungal structures.