The distribution of heavy ozone (molecular mass 50) has been measured for the first time in the stratosphere with a mass spectrometer system. During a balloon descent after midnight on Sept. 4, 1980, a pronounced enhancement of heavy ozone was found. The maximum in the isotopic ratio, 1801602/1603, occurred at 32 km where the enhancement of heavy ozone was over 40%. The ratio decreased toward higher and lower altitudes, and reached the standard value below 24 km. These measurements confirm an earlier prediction that there exists a preferred production mechanism for heavy ozone in the stratosphere.