Abstract Effects of CO2 level (ambient vs. elevated) on the interactions among three cotton (Gossypium hirsutum) genotypes, the cotton aphid (Aphis gossypii Glover), and its hymenoptera parasitoid (Lysiphlebia japonica Ashrnead) were quantified. It was hypothesized that aphid-parasitoid interactions in crop systems may be altered by elevated CO2, and that the degree of change is influenced by plant genotype. The cotton genotypes had high (M9101), medium (HZ401) and low (ZMS13) gossypol contents, and the response to elevated CO2 was genotype-specific. Elevated CO2 increased the ratio of total non-structural carbohydrates to nitrogen (TNC: N) in the high-gossypol genotype and the medium-gossypol genotype. For all three genotypes, elevated CO2 had no effect on concentrations of gossypol and condensed tannins. A. gossypii fitness declined when aphids were reared on the high-gossypol genotype versus the low-gossypol genotype under elevated CO2. Furthermore, elevated CO2 decreased the developmental time of L. japonica associated with the high-gossypol genotype and the low-gossypol genotype, but did not affect parasitism or emergence rates. Our study suggests that the abundance of A. gossypii on cotton will not be directly affected by increases in atmospheric CO2. We speculate that A. gossypii may diminish in pest status in elevated CO2 and high-gossypol genotype environments because of reduced fitness to the high-gossypol genotype and shorter developmental time of L. japonica.