A large seasonal cycle has been observed in the 13C/12C isotopic ratio of CO in clean air in the extratropical southern hemisphere. This ratio is determined by the mixture of CO from isotopically distinct sources and is strongly influenced by the relative contributions of surface sources and the oxidation of CH4. We use a zonally averaged atmospheric model to relate atmospheric CO mixing ratios and 13C/12C isotopic ratios to the magnitude and distribution of CO sources and to explain the average seasonal cycles observed. The average 13C/12C ratio of CO emitted by surface sources in the southern hemisphere is larger than in the northern hemisphere and the southward flux of CO into the extratropical southern hemisphere is additionally enriched in 13C as a result of oxidation during transport. These effects partially offset the effect of highly depleted 13C/12C ratios in CO produced by CH4 oxidation. In the extratropical southern hemisphere, seasonal variation in the fraction of atmospheric CO derived from CH4 oxidation produces large changes in 13C/12C which are partially offset by seasonal variations of surface sources. A good fit to observed average cycles can be obtained using surface source strengths consistent with previous estimates. However, the southern hemisphere data place strong constraints on the CH4 oxidation source and imply that either the CO yield per molecule of CH4 is about 0.7, compared with previous estimates of around 0.8, or that unidentified processes associated with CH4 oxidation cause 13C enrichment of about 4? in the CO produced.¿ 1997 American Geophysical Union