Tropospheric ozone samples collected during a twelve-month period in urban air show an enrichment of about 9% in the heavy isotope 50O3 consistent with predictions from laboratory measurments. The enhancement of about 7% observed in 49O3 is still within the uncertainty of the expected value. These measurements confirm that the isotope effect, repeatedly found in laboratory experiments, is also produced in the atmosphere during the ozone formation process. ¿ American Geophysical Union 1995