Plant growth and yield responses to carbon dioxide (CO₂) enrichment are well established. Much less is known of the response of arthropod pests to CO₂ enrichment. Reproductive response of twospotted spider mites (Tetranychus urticae Koch) on white clover (Trifolium repens L.) to a range of CO₂ concentrations was measured. The CO₂ treatments were applied for 24 h d⁻¹ at ≈395, 484, 570, 657, and 748 μLL⁻¹ on the 14 d before and 26–27 d after infestation with mites. Eggs, larvae, nymphs, and adult mites were removed from leaves and counted 27–29 d after infestation. Leaf area and weight were measured, and leaves were analyzed to measure structural and nonstructural carbohydrates, N, amino acids and digestibility. Carbon dioxide enrichment caused linear increases in plant growth and foliar nonstructural carbohydrates, but caused linear decreases in foliar N. Carbon dioxide enrichment significantly increased the rate of mite reproduction on both clover clones. Correlations between mite population increase were significantly positive for foliar nonstructural carbohydrates and significantly negative for foliar N. Concentrations of ambient CO₂ expected in the 21st century may increase the risk of mite population damage on some plant species.