Kim and Craig [1993] have shown that N2O in the stratosphere is enriched in both 18O and 15N relative to tropospheric N2O. It is difficult to account for this result in light of the measurements of Selwyn and Johnston [1981] and Johnston et al. [1995] indicating that isotopic fractionation associated with the known stratospheric sinks, photolysis and reaction with O(1D), is minimal. It is suggested that the puzzle could be resolved by invoking an additional source of isotopically heavy N2O formed perhaps by reaction of N2 with CO3 produced by recombination of O(1D) and CO2. This source would require a significant reduction in estimates for net, mainly biological, emission of N2O at the surface. ¿ American Geophysical Union 1996