The extratropical stratosphere-troposphere S/T exchange is investigated on isentropic surfaces that intersect the tropopause, using a semi-Lagrangian transport model and analyzed winds from the European Center for Medium-Range Weather Forecasts (ECMWF). It is found that the characteristics of extratropical S/T exchange exhibit two distinctive regimes. On the 330 K isentropic surface and below, S/T exchange occurs vigorously in all seasons, mainly caused by the irreversible mixing and transport by breaking synoptic-scale baroclinic disturbances. However, on and above the 340 K isentropic surface, S/T exchange exhibits a strong annual cycle where very little S/T exchange takes place in the winter hemisphere, but significant S/T exchange occurs in the summer hemisphere, particularly in the northern summer. It is argued that the weak S/T exchange in the winter hemisphere is mainly due to the barrier effect of the strong potential vorticity (PV) gradient at the tropopause and the active S/T exchange in the summer hemisphere is mainly associated with the summer Asian and Mexican monsoons in the northern summer and the Australia and South American monsoons in the southern summer. ¿ American Geophysical Union 1995