Technical Abstract: Invasive plants are considered to be a major threat to the Earth's biodiversity and are estimated to cost U.S. agricultural and forest producers 34 billion dollars each year from decreased productivity and increased costs of production for weed control. While considerable effort is being spent studying these exotic plant pests, little consideration of how invasive plants might react to the increasing carbon dioxide (CO2) concentration in the atmosphere has been given. We have been growing a variety of invasive plants under ambient and elevated levels of CO2. To date, study has concluded for two species (Johnson grass [Sorghum halepense] a C4 grass and sicklepod [Cassia obtusifolia] a C3 N2-fixing legume). Results show that, while both plants grew larger under the high level of CO2, the response of sicklepod was much greater (average increase in aboveground dry weight = 37%) than did Johnson grass (20%). In addition, both species showed a delay or decrease in reproduction when exposed to elevated CO2 as evidenced by reduction in both numbers and dry weights of reproductive structures. These results suggest that, while invasive weeds may grow larger under a future, high CO2 environment, their ability to spread might be reduced.