We traced the presence and distribution of Neotyphodium lolii within developing inflorescences and embryos of perennial ryegrass (Lolium perenne L.), cultivar "Grassland Nui," by in situ immunolocalization of fungal proteins. Evidence is presented that the fungus penetrates through the rachilla at the base of the ovary, and localizes in a very precise and specific manner in the ovular nucellus, but never enters the embryo sac or the integuments. Young embryos do not contain mycelium, but as they mature the hyphae penetrate through the scutellum from a neighboring "infection layer"-a remnant of nucellus heavily colonized by the fungus-that is readily visible as a discrete area directly adjoining the base of embryo cavity. Our observations document that N. lolii is transmitted to the embryo exclusively via sporophytic maternal tissue.