High O3 mixing ratios were observed above 6 km over Syowa Station (SYO), Antarctica in mid-September 1989 when the tropopause was found to be higher than 10 km. From the results of a 3-dimensional trajectory analysis, it is suggested that the observed high O3 was caused by vertical transport of O3 from the lower stratosphere to the upper troposphere. To examine general features of the transport process associated with high O3 mixing ratios observed in the upper troposphere over SYO, the same trajectory analysis was further performed for high O3 observations at 350 hPa over SYO in 1987--1994. The results imply that the transport of stratospheric air descending in the polar region to SYO plays a very important role in the increase of upper tropospheric O3 over SYO, especially in spring and autumn, while the downward transport of stratospheric O3 in the circumpolar region is important in summer. ¿ 1998 American Geophysical Union