Tropospheric ozone events observed at the summit of Mount Fuji (3776 m above sea level) were analyzed as case studies. The ozone, intruding from the lower stratosphere by the cutoff low or the tropopause fold over the Asian continent, is transported to the middle troposphere over Japan even in summer. The subtropical jet, which is intensified over Japan, also contributes to the summer intrusion from the stratosphere. A long stratospheric streamer, as described previously by Appenzeller et al. <1996>, brings about a persistent ozone enhancement at the summit of Mount Fuji. The large variation of summer ozone over Japan is attributable to the alternate overspreading of these ozone-rich stratospheric air masses and the ozone-depleted subtropical maritime air mass. In contrast, winter ozone variation is relatively small at the summit of Mount Fuji. The steep potential temperature gradient between 500 hPa and 300 hPa in the winter cutoff low, which restrains vertical mixing in the upper troposphere, possibly causes less influence of the stratospheric air mass on the middle troposphere in winter, since mixing processes around a cutoff low play an important role in air mass exchange between the stratosphere and the troposphere. ¿ 1998 American Geophysical Union