Abstract: The growth characteristics of alligator weed, Alternanthera philoxeroides, and the larvae development and adult feeding habit of Agasicles hygrophila under elevated atmospheric CO₂ were investigated.Three CO₂ concentrations, i.e., 420, 550 and 750 μL·L^-1, were applied in a closed chamber for the experiment.It was found that A.philoxeroides grew faster at higher CO₂ concentrations.On the 42^nd day, the heights of the weeds were 35.60 cm and 40.04 cm at 550 μL·L^-1 CO₂ and 750 μL·L^-1 CO₂, respectively.As compared to that of control (19.83 cm), the weed height was 1.75 times higher under 550 μL·L^-1 CO₂, and two times higher under 750 μL·L^-1 CO₂.In an elevated CO₂ environment, the leaves of the weeds showed increases on the SPAD, photosynthesis, soluble sugars, and C/N ratio, but decreases on the nitrogen-containing compounds (such as, proteins and amino acids) and the tannin content.When fed on A.philoxeroides under elevated CO₂, the weight of A.hygrophila larvae was higher than that of control, while the emerged adults, male or female, appeared to prefer A.philoxeroides grown under 750μL·L^-1 CO₂.