Forage grasses (e.g., Lolium spp. andFestuca spp.) have long been known to harbour fungi which maintain an intercellular relationship with leaf tissue of the host. These endophytic fungi are considered to be the source of toxins which accumulate in infected grasses and are the cause of physiological disorders in grazing sheep and cattle. On the other hand, these same endophytes contribute to the resistance of the host plant to certain pests. Few investigators have examined the extent to which the fungal endophyte invades different parts of the grass. Our studies have recently concentrated on the mechanism by which the endophytes ofF. arundinacea andF. versuta invade the embryo and thus establish residence in successive host generations.