Stratospheric photochemistry leads to anomalous oxygen isotope enrichments in CO2 (for which Δ17O = δ17O - 0.516 ¿ δ18O ≠ 0). This isotope anomaly is not lost until air returns to the troposphere and CO2 undergoes isotope exchange with water primarily in the terrestrial biosphere and oceans. A two-box model is used to investigate the contribution of stratospheric production and contemporary surface carbon fluxes to tropospheric Δ17OCO2. The predicted magnitude of ~0.15? is large enough that measurement of a globally averaged tropospheric Δ17OCO2 should provide a new constraint for gross carbon exchanges between the biosphere and atmosphere in terrestrial carbon cycle models. Importantly, Δ17OCO2 should be complementary to the primary isotopic tracer of gross carbon exchanges, δ18OCO2, but is not dependent on numerous hydrologic variables. Furthermore, with improved measurement precision, Δ17OCO2 could serve as a direct tracer of gross carbon exchanges and their variations.