A protein fraction, previously isolated from the lichenXanthoria parietina and known to bind to the appropriate culturedTrebouxia phycobiont, was visualized in the intact lichen thallus and cultured mycobiont by an indirect immunoperoxidase assay. The protein was localized in both the upper and lower cortices of the lichen thallus; it was also present in the cell walls of the mycobiont culturedin vitro. The possible role of this protein in the recognition, or initial interaction, between separated lichen symbionts is discussed.