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1. The performance of foliage feeders tends to decrease under elevated CO2, but the responses of phloem-feeding insects have been much more equivocal. As phloem tissues are less accessible than whole-plant tissues, much less is known about how phloem composition is altered under elevated CO2 and the mechanisms driving changes in aphid performance. 2. In this study, the plant mechanisms underlying the performance of Rhopalosiphum padi aphids on Hordeum vulgare (barley) grown under ambient (390 ppm) and elevated (700 ppm) CO2 were examined. We used aphid stylectomy to sample pure phloem from plants in CO2-controlled conditions and high-performance liquid chromatography to analyse phloem samples for amino acid concentrations. 3. Aphid abundance significantly increased by 127% under elevated CO2. Consequently, plant biomass decreased under elevated CO2 in trials with herbivores present, possibly due to the increased herbivore load, but increased when aphids were absent. The intrinsic rate of population increase ( rm) was significantly higher under elevated CO2; however, there were no statistically significant effects on aphid fecundity or development time. The concentration of individual amino acids tended to increase, although these increases were statistically significant in only a few cases. A principal components analysis revealed that the relative abundance (mol %) of those amino acids considered essential for aphids tended to increase under elevated CO2. 4. These results indicate that CO2 may affect nutrient translocation in plants in ways that are contrary to predictions about nitrogen metabolite responses to CO2. Such plant biochemical responses may underlie observations of improved phloem feeder performance under elevated CO2. [ABSTRACT FROM AUTHOR]