Stratospheric air samples were collected over Japan on August 31, 1994 using a balloon-borne cryogenic sampler and then analyzed for the CH4 concentration and Δ13C in CH4. The Δ13C value increased from about -47.5? at the tropopause to about -38.9? at 34.7 km, accompanied by a rapid decrease of the CH4 concentration with height. From the measured values of the CH4 concentration and Δ13C, the apparent carbon isotopic fractionation factor was estimated to be 1.0131¿0.0006. The contributions of the chemical CH4 reactions with OH, O(1D) and Cl were also examined by using an one-dimensional photochemical-diffusion model. It is suggested from this examination that the reaction of CH4 with Cl is especially important for Δ13C in the stratosphere. ¿ 1997 American Geophysical Union