The effects of mixtures of ozone and carbon dioxide on growth and physiology of an O3-sensitive (NC-S) and an O3-resistant (NC-R) clone of white clover (Trifolium repens L.) were determined. The experiment was performed in a greenhouse with O3 treatments of 5 and 82 nl l−1 (ppb) for 6 h d−1 and CO2 treatments of 380 (ambient), 490,600, and 710 μl l−1 (ppm) for 24 h d−1. Enrichment with CO2 decreased foliar gas exchange (measured as stomatal resistance) of NC-R more than that of NC-S whereas O3 decreased gas exchange of NC-S more than that of NC-R. Ozone caused extensive foliar injury of NC-S but caused only slight injury of NC-R. CO2 enrichment suppressed O3-induced foliar injury of NC-S as measured after 4 wk of exposure, but this effect diminished after 8 wk of exposure. CO2 enrichment decreased the relative chlorophyll content (μg of chlorophyll mg−1 of leaf tissue sampled) but not the total chlorophyll (total chlorophyll in the leaves sampled). There were no O3× CO2 interactions for foliar chlorophyll. High concentrations of CO2 caused reddening of new leaves near the end of the 8 wk exposure period. CO2 enrichment decreased foliar concentrations of N, P, K, S, Cu, B, and Fe, increased foliar concentrations of Mn, but did not affect Zn, Ca, or Mg. Ozone exposure did not modify the CO2 effects on foliar nutrient concentration. Ozone decreased growth of NC-S but not NC-R while CO2 enrichment stimulated growth of both clones. The highest CO2 concentration appeared to decrease the effects of O3 on growth of NC-S. However, except for a transitory effect on foliar injury, there was no evidence that CO2, at concentrations less than the highest used in this study, will protect white clover from the effects of tropospheric O3.