Recent investigations of the carbon kinetic isotope effect (KIE) of the reaction CH4+Cl yielded KIECl=1.066¿0.002 at 297 K (increasing to 1.075¿0.005 at 223 K) [Saueressig et al., 1995>. In order to assess the effect of the exceptionally large KIECl on Δ13C of stratospheric CH4 we applied a two-dimensional, time dependent chemical transport model. The model results demonstrate the strong influence of the CH4+Cl reaction on Δ13CH4 in particular in the middle and upper stratosphere, where this reaction contributes several tens of percent to the total CH4 sink. The Cl sink helps to explain the relatively large overall isotope fractionation of 1.010--1.012 observed in the lower stratosphere [Brenninkmeijer et al., 1995; Brenninkmeijer et al., 1996>, even though the model results predict a smaller effect than observed. ¿ American Geophysical Union 1996