Individual tall fescue (Festuca arundinacea) plants infected with both Neotyphodium coenophialum and N. lolii endophytes, and individual perennial ryegrass (Lolium perenne) plants infected with both N. lolii and Neotyphodium LpTG-2, were obtained following inoculation of naturally infected seedlings with the second endophyte. Differences in the ability of the endophytes to produce conidia, together with colony characteristics, enabled the endophytes in plants to be identified following incubation of excised leaf tissue on potato dextrose agar. Most tillers of dually infected plants were infected with just a single endophyte, but tillers infected with two endophytes were identified in three tall fescue plants. In these tillers one endophyte was always present at a much higher concentration than the other. Over time the incidence of dually infected tillers decreased and no tillers with both N. coenophialum and N. lolii were present 5 months after the associations were established. No evidence was obtained of exchange of nuclei between different endophytes present in dually infected tillers giving rise to heterokaryons or interspecific hybrids.