Ozone measurements near 200 mbar from two flights between California and Tahiti are interpreted using maps of potential vorticity (PV) on isentropic surfaces. We focus on extremely abrupt changes in ozone mixing ratio observed at latitudes of 13 ¿N and 23.5 ¿N. Their proximity to strong PV gradients on the 350 K isentropic surface shows that they are associated with crossings of the subtropical tropopause. Small-scale anticorrelations between ozone and carbon dioxide near one of the two ozone transitions indicate that some stratosphere-troposphere exchange does occur in this region. Ozone mixing ratios on the stratospheric side of the subtropical tropopause varied from 50 to 100 parts per billion by volume, a range that is more commonly associated with the midlatitude troposphere and is much less than seen on the stratospheric side of the midlatitude tropopause. ¿ American Geophysical Union 1996