Rising global carbon dioxide levels may lead to profound changes in plant composition, regardless of the degree of global warming that may result from the accumulation of this greenhouse gas. We studied the interaction of a CO2 doubling and two levels of nitrogen fertilizer on the growth and chemical composition of tall fescue (Festuca arundinacea Schreber cv. KY-31) when infected and uninfected with the mutualistic fungal endophyte Neotyphodium coenophialum Morgan-Jones and Gams. Two-year-old plants were harvested to 5 cm every 4 weeks, and after 12 weeks of growth plants grown in twice ambient CO2 concentrations: photosynthesized 15% more; produced tillers at a faster rate; produced 53% more dry matter (DM) yield under low N conditions and 61% more DM under high N conditions; the % organic matter (OM) was little changed except under elevated CO2 and high N when %OM increased by 3%; lignin decreased by 14%; crude protein (CP) concentrations (as %DM) declined by 21%; the soluble CP fraction (as %CP) increased by 13%; the acid detergent insoluble CP fraction (as %CP) increased by 12%, and in vitro neutral detergent fiber digestibility declined by 5% under high N conditions but not under low N. Plants infected with the endophytic fungus: photosynthesized 16% faster in high N compared with under low N; flowered earlier than uninfected plants; had 28% less lignin in high N compared with under low N; and had much smaller reductions in CP concentration (as %DM) and smaller increases in the soluble CP fraction (as %CP) and the acid detergent insoluble CP fraction (as %CP) under elevated CO2. Such large and varied changes in plant quality are likely to have large and significant effects on the herbivore populations that feed from these plants.