Background/Question/Methods Rising atmospheric CO2 causes distinct changes in organism physiology and phenotype, which may affect interactions among organisms. Recent work demonstrates that aboveground organisms can substantially affect belowground interactions. How does environmental change, specifically increasing atmospheric CO2, affect above-belowground interactions? We examined the effect of elevated CO2 and two specialist herbivore species on belowground defensive chemistry and mycorrhizal interactions. We grew seedlings from 5 genetic families (genotypes) of Asclepias syriaca, inoculated them with their native soil community, and grew all plants in open-top chambers, exposing half of the plants to elevated CO2 (770 ppm), and the other to ambient CO2 (385 ppm). Plants were grown for 3 months under CO2 treatment and protected from herbivory during this time. After 3 months, half of the plants received 5 days of herbivory from one of two specialist herbivores, Aphis asclepiadis or Danaus plexippus. Both herbivore-treated and control plants were harvested after the herbivore treatment, and plant defenses, mycorrhizal colonization, and mycorrhizal community composition were assessed. We measured root cardenolide concentration using UPLC, mycorrhizal colonization by staining root subsections, and mycorrhizal community composition was analyzed using terminal restriction fragment length polymorphisms (T-RFLP). Results/Conclusions We found that herbivory by aphids, but not monarch larvae, decreased cardenolide concentration in fine roots. Elevated CO2 mitigated this decline in a single cardenolide compound, but did not affect induced declines in any other cardenolide compound belowground. Herbivory by caterpillars, but not by aphids, increased mycorrhizal colonization and the percent root length colonized by arbuscules. Elevated CO2 increased the percent of root colonized by vesicles and increased colonization by hyphae and arbuscules after aphid herbivory in all but one plant genotype. Initial T-RFLP analysis indicates that herbivory by caterpillars and aphids tended to increase the diversity of mycorrhizal fungi (fragment lengths) found in A. syriaca roots. We conclude that elevated CO2 alters above-belowground interactions by mitigating declines in defense expression of specific cardenolide compounds as well as increasing colonization by mycorrhizal fungi in plants experiencing aphid herbivory.