Analyses of the CO stable isotopic composition in surface air samples collected at high northern latitudes during Arctic spring reveal significant depletions in the 13C content of atmospheric CO, which coincide with episodes of tropospheric O3 loss. This isotope signal can be explained by production of small amounts of highly 13C-depleted CO, formed in Cl-initiated oxidation of CH4 due to the presence of enhanced Cl levels during O3 depletion events. Using the recently measured fractionation factor for the CH4+Cl reaction, the observed 13C depletion is employed to estimate the time-integrated amount of Cl radicals encountered by the O3 depleted air mass. The results support findings obtained from measurements of hydrocarbon mixing ratios during O3 depletion events. The 13C content of atmospheric CO can thus be used as a sensitive indicator for enhanced Cl radical levels in the atmosphere. ¿ 1999 American Geophysical Union