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We tested whether the efficacy of chemical weed control might change as atmospheric CO₂ concentration [CO₂] increases by determining if tolerance to a widely used, phloem mobile, postemergence herbicide, glyphosate, was altered by a doubling of [CO₂]. Tolerance was determined by following the growth of Amaranthus retroflexus L. (redroot pigweed), a C₄ species, and Chenopodium album L. (common lambsquarters), a C₃ species, grown at near ambient (360 μmol mol⁻¹) and twice ambient (720 μmol mol⁻¹) [CO₂] for 14 d following glyphosate application at rates of 0.00 (control), (0.112 kg ai ha⁻¹) (0.1 × the commercial rate), and 1.12 kg ai ha⁻¹ (1.0 × the commercial rate) in four separate trials. Irrespective of [CO₂], growth of the C₄ species, A. retroflexus, was significantly reduced and was eliminated altogether at glyphosate application rates of 0.112 and 1.12 kg ai ha⁻¹, respectively. However, in contrast to the ambient [CO₂] treatment, an application rate of 0.112 kg ai ha⁻¹ had no effect on growth, and a 1.12-kg ai ha⁻¹ rate reduced but did not eliminate growth in elevated [CO₂]-grown C. album. Although glyphosate tolerance does increase with plant size at the time of application, differences in glyphosate tolerance between CO₂ treatments in C. album cannot be explained by size alone. These data indicate that rising atmospheric [CO₂] could increase glyphosate tolerance in a C₃ weedy species. Changes in herbicide tolerance at elevated [CO₂] could limite chemical weed control efficacy and increase weed-crop competition.