Isotope and concentration measurements are reported for CO and CH4 in air collected in the upper troposphere and lower stratosphere between New Zealand and Antarctica in October 1993. The 13C/12C ratio of CO for the stratospheric samples, which are identified using calculated potential vorticity and coherent isotope data, like the abundance of 14CO molecules, are much lower than all previously reported atmospheric values. The measurements manifest a very steep decrease in Δ13C with declining CO, with one sample reaching a Δ13C value relative to V-PDB of -43? at 20 ppbv CO. This large isotope shift is caused by the local production of several ppbv of extremely depleted CO. Not only is CH4 itself a 13C depleted precursor of CO, it is specifically the recently discovered large fractionation in Cl+CH4, and the availability of free Cl during ozone hole conditions, which causes the effect. ¿ American Geophysical Union 1996