Carbon dioxide (CO₂) enrichment in the atmosphere stimulates photosynthetic activity and growth of C₃ plants. This may in turn alter the availability of photosynthates for plant-associated microbes, modifying the symbiosis formed such as mycorrhizae and plant-endophyte complexes. Documents are accumulating to show that elevated CO₂ increases hyphal growth and root colonization by arbuscular mycorrhizal fungi (AMF). Similar to AMF, endophytes are also fungi that are widely associated with plants but they mostly exist in the shoots rather than the roots of plants. Up to now, however, few studies have focused on the responses of endophyte-infected plants to elevated CO₂. In the present study, we examined how elevated CO₂ affects endophytes and their functions, using Achnatherum sibiricum (L.) Keng as model species. A. sibiricum is a caespitose perennial grass, widely distributed in the Inner Mongolia steppe and usually highly infected by Neotyphodium endophytes. Seeds of A. sibiricum were collected from natural population in Hailar in the Northeast part of China. Detection of endophytes using the aniline blue staining method showed that endophyte infection frequency of the Hailar population was almost 100%. To eliminate the endophyte, we heat-treated a subset of randomly chosen seeds in a convection drying oven for 30 d at 60 ℃. Two experiments were performed in two growth chambers, with ambient (C-) and elevated (C+) CO₂, separately. In Experiment 1, germination rates of endophyte-infected (E+) and endophyte-free (E-) seeds were compared under two different CO₂ concentrations. In Experiment 2, vegetative growth of E+ and E-seedlings was compared. The design of this experiment was completely randomized and a 2×2×3 factorial, with CO₂ concentration (C+ vs. C-), infection status (E+ vs. E-) and nutrients availability (N+P+, N-P+, N+P-, i.e. N and P supply, N deficiency P supply, N supply P deficiency) as the variables. There were five replicates per treatment group. The results showed that both the germination rate and germination speed of E+ seeds were not affected by elevated CO₂ while those of E-seeds were significantly decreased by elevated CO₂. That is to say, elevated CO₂ increased the germination rate difference between E+ and E-seeds. Endophyte infection significantly improved maximum net photosynthetic rate and water use efficiency of the host grass. The vegetative growth was significantly affected by the interaction of elevated CO₂ and nutrients availability, but was not affected by endophyte infection. The beneficial improvement of elevated CO₂ on vegetative growth of A. sibiricum occurred only under N+P+ conditions. With N or P deficiency, the beneficial effect of elevated CO₂ on the growth did not exist. The root morphological characters were affected by the interaction of elevated CO₂ and endophyte infection. In the ambient CO₂ treatment, the proportion of root length with a diameter of >1.05 mm was significantly higher in E+ than in E-plants. With elevated CO₂, no significant difference was found in the proportion of the root length stated above between E+ and E-roots. Elevated CO₂ decreased the difference of root morphology between E+ and E-plants. When compared with plant-AMF associations, the present study suggested that the grass-endophyte association was less sensitive to CO₂ enrichment. It is suggested that more experiments are needed to fully examine the potential impacts of elevated CO₂ on plant-endophyte associations.