Regular ozonesonde observation and total ozone observation with the Brewer spectrophotometer have been conducted at Watukosek (7.5 ¿S, 112.6 ¿E), Indonesia, since 1993. Three seasons are recognized for the vertical distribution of tropospheric ozone. (1) During the local wet season, between December and March, the ozone mixing ratio is nearly constant at 25 ppbv throughout the troposphere. (2) During the transition season from wet to dry, between April and July, the mixing ratio is often enhanced in the uppermost troposphere. (3) During the local dry season, between August and November, the concentration is enhanced in the planetary boundary layer, and extensive forest fires in Indonesia associated with the strong El Ni¿o events of 1994 and of 1997 have enhanced the ozone mixing ratio in the middle troposphere, the integrated tropospheric ozone, and the total ozone at Watukosek. ¿ 2000 American Geophysical Union