Measurements of the triple oxygen isotopic composition of stratospheric CO2 in whole air samples from the NASA ER-2 aircraft show anomalous enrichments in 17O and 18O. The compact correlation of the isotope anomaly (defined as Δ17O = δ17O - 0.516 ¿ δ18O) with simultaneous N2O measurements demonstrates that Δ17OCO2 is a long-lived tracer with a stratospheric source. These characteristics, and an isotopic link to O3 production, make Δ17OCO2 potentially useful as a tracer of integrated stratospheric chemistry and transport. The Δ17OCO2:N2O correlation is also used to estimate a net Δ17OCO2 flux to the troposphere of 3.6 ¿ 0.9 ¿ 1015? mol CO2 yr-1. This flux is required to predict and understand the CO2 and O2 isotope anomalies in the troposphere and their use as tracers of gross carbon exchanges between the atmosphere and biosphere on interannual to glacial-interglacial time scales.