High CO2 was applied to field‐grown winter wheat (Triticum aestivum L. cv. Arthur) during different growth periods in order to identify when photosynthesis was most limiting grain yield. Wheat was grown in open top Mylar® polyester chambers which were provided with circulated air (340 μL/L CO2 = control treatment) or air enriched with sufficient CO2 to expose the foliage to 1200 μL/L (high CO2 treatment). The CO2 enrichment treatments were applied from jointing to anthesis, from jointing to maturity, or from anthesis to maturity. Seed yield and total dry matter were influenced most by CO2 during the period from jointing to anthesis. The 17% increase in seed yield resulted from more seed per meter2, which was associated with an increase in heads per meter2. Total dry matter at maturity was increased by 11%. Neither harvest index nor nitrogen content of plant parts at harvest were altered by CO2 enrichment. In order to characterize the effect of CO2 on seed yield, several physiological parameters of leaf function were monitored using the flag leaves of plants grown in the control chambers and the high CO2 chambers treated from jointing to maturity. High CO2 had no significant effect on flag leaf chlorophyll and protein content or the onset of their decline during senescence. In addition, there was no effect on the level of ribulose bisphosphate carboxylase or its decline during senescence or on total proteolytic activity. High CO2 did, however, cause a 50% increase in apparent photosynthesis of the flag leaf. This resulted in a doubling of the sucrose and starch content of the flag leaf prior to seed growth. With the onset of seed growth this difference in sucrose and starch levels rapidly disappeared, suggesting a rapid mobilization to the head and developing seeds.