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Vertically transmitted endophytic fungi can mitigate the negative effects of salinity encountered by their host grass and alter the competitive interactions between plant individuals. To experimentally study the interactive effects of the fungal endophyte <i>Epichloë coenophiala</i> on salt tolerance and intraspecific competition of its host plant, tall fescue <i>Festuca arundinacea</i>, we subjected 15 maternal lines of each <i>Epichloë</i> associated (E+) and <i>Epichloë</i> free (E−) tall fescue to salt treatment and competition in the greenhouse and common garden. Then, to explore variation in endophyte incidence in natural populations of tall fescue, we surveyed 23 natural populations occurring on or near the Baltic Sea coast in Aland islands in southwestern Finland for endophyte incidence, distance to shore, and competitive environment. Under salinity in the greenhouse, E− plants grew larger than E+ plants, but there was no size difference in the control treatment. E− plants grew taller and were more likely to flower than E+ plants when grown in benign conditions in the common garden but not with salinity or competition. The frequency of <i>Epichloë</i> incidence was high (90%) in natural populations, and it decreased towards the shore and risk of salt exposure. These results demonstrate a negative effect of <i>Epichloë</i> endophyte on the salt tolerance of its host. The high incidence of <i>Epichloë</i> in natural populations of tall fescue in the northern part of the species distribution range is likely due to factors other than salinity.