SUMMARYThe development of Pezizella ericae Read in living roots of host plants is typically limited to cells surrounding the central cylinder and the biotrophic phase of host-fungus interactions is characterized by a highly compatible relationship between the symbionts at the cellular level. In non-host plants, the ericoid endophyte extensively colonizes root tissues, including the central cylinder, without forming hyphal coils and a necrotrophic relationship occurs at the cellular level, the fungus provoking a rapid degeneration of the contents of plant cells. A fundamental difference between the interactions occurring in cells of host and non-host roots is the constant presence of the plant plasmalemma surrounding P. ericae in the former and the complete lack of this membrane around hyphae in the latter. External hyphae of P. ericae develop an abundant fibrillar sheath in the presence of both host and non-host roots. This disappears with hyphal development in living host cells but it persists around intracellular hyphae in non-host tissues. These observations are discussed in relation to fungus-plant recognition phenomena and the control of infection in ericoid mycorrhizas.