The D/H kinetic isotope effect (KIEClD) in the reaction of CH4 with Cl has been investigated using a tunable diode laser absorption spectrometer (TDLAS) to measure 12CH3D/12CH4 ratios. A KIEClD of 1.508¿0.041 was obtained at room temperature (296 K). Experiments employing Fourier transform infrared measurements of 12H3D/12CH4 ratios confirmed this result. An investigation of the temperature dependence showed an increase of KIEClD (1.592¿0.057 at 223 K) with decreasing temperature. This very large D/H fractionation effect affects the overall isotope fractionation in the middle and upper stratosphere, where Cl contributes substantially to the total removal of CH4. ¿ American Geophysical Union 1996