The vertical distribution of isotopomers of N2O (14N15N16O, 15N14N16O, and 14N14N18O) in the lower and middle stratosphere was observed over Japan (39 ¿N, 142 ¿E) in 1999 using a balloon-borne cryogenic sampler and ground-based mass spectrometry. The abundance of the heavier isotopomers relative to 14N14N16O increased with altitude, while the mixing ratio of N2O decreased due to photochemical depletion. Maximum fractionation was observed at the highest altitude, 34.5 km, where Δ15Nairα (isotopomer ratio of center nitrogen), Δ15Nairβ (end nitrogen), and Δ18OSMOW were 144.1?, 42.4?, and 119.0?, respectively. The observed distribution is mostly accounted for by isotopic fractionation during consumption processes, which is in accordance with reported simulation experiments and theoretical prediction for photolytic fractionation of N2O isotopomers. However, the apparent fractionation factors in the lower (<~24 km) and higher regions are different, which suggests that (1) the fractionation factor for stratospheric photolysis may depend on altitude or latitude, (2) transport and mixing processes in the stratosphere can affect the vertical profile, and (3) the relative contribution of photolysis and photo-oxidation to total N2O sink is possibly dependent on altitude. ¿ 2001 American Geophysical Union