Allocation of allomones of transgenic Bacillus thuringiensis Gossypium hirsutum (Bt cotton) (cv. GK-12) and non-Bt-transgenic cotton (cv. Simian-3) grown in elevated CO(2) in response to infestation by cotton aphid, Aphis gossypii Glover, was studied in a closed-dynamics CO(2) chamber. Significant increases in foliar condensed tannin and carbon/nitrogen ratio for GK-12 and Simian-3 were observed in elevated CO(2) relative to ambient CO(2,) as partially supported by the carbon nutrient balance hypothesis, owing to limiting nitrogen and excess carbon in cotton plants in response to elevated CO(2). The CO(2) level significantly influenced the foliar nutrients and allomones in the cotton plants. Aphid infestation significantly affected foliar nitrogen and allomone compounds in the cotton plants. Allomone allocation patterns in transgenic Bt cotton infested by A. gossypii may have broader implications across a range of plant and herbivorous insects as CO(2) continues to rise.