Fungal endophytes of some cultivated grasses can increase plant performance and competitive abilities, especially under stress. Far less is known about the influence of <i>Epichloë</i> infections in wild populations of wild grasses. In this study, plants of three <i>Elymus dahuricus</i> ecotypes (WLS, QY and WTS) either infected with <i>Epichloë</i> endophyte (E+) or uninfected (E–) were grown in the field. The activities of the antioxidant enzymes ascorbate peroxidase, catalase, peroxidase and superoxide dismutase, and concentrations of H<sub>2</sub>O<sub>2</sub> and malondialdehyde were examined in the leaves of E+ and E– plants. We also determined photosynthesis parameters, leaf blade and sheath carbohydrate concentration and plant growth parameters of both E+ and E– plants. E+ plants from the WLS and QY populations had significantly higher antioxidant enzyme activities and photosynthetic capability (<i>P</i><0.05), superior growth characteristics including more abundant carbohydrate concentration than E– plants. In contrast, in plants from the WTS population, the endophyte had no significant effect on reactive oxygen species scavenging capability and growth performance (<i>P</i>>0.05), and even displayed some negative effects on plant photosynthetic capability. Thus, endophyte infection significantly affected <i>E. dahuricus</i> antioxidant enzyme activities (<i>P</i><0.05), photosynthesis and growth capability, although, the effects varied with plant ecotypes.