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 X the commercial rate), and 1.12 kg ai ha−1 (1.0 X 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 tolerant 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 limit chemical weed control efficacy and increase weed–crop competition.