Enrichment of heavy O3 isotopes has been measured in collected stratospheric samples. A balloon-borne cryogenic sampler was used to gather six O3 samples between 26 and 35 km in three flights. Subsequent laboratory mass spectrometer analysis of rare O3 isotopes at both mass 49 and 50 has resulted in more precise measurements than have previously been reported with in situ and ground-based techniques. In one flight, 50O3 was enriched by 12--16% and 49O3 by 9--11%, both increasing with altitude. In the remaining two flights, the isotope enrichment was nearly mass-independent at 8--9%. The enrichments in O3 at mass 50 are less than the large 40% value observed in some stratospheric measurements but similar to 49O3 and 50O3 fractionations produced in laboratory-generated ozone. ¿ American Geophysical Union 1990