A global chemical transport model, called MOZART, is used to investigate the photochemical impact of the 1997 Indonesian fires on tropospheric ozone and its precursors in the tropics. Due to the high release of carbon monoxide by peat fires, CO increases by up to 1000 ppbv in the free troposphere over Indonesia. As a consequence of increased photochemical production, ozone is significantly perturbed over source regions (Sumatra and Kalimantan). The tropospheric O3 column increases by 20--25 DU and the ozone mixing ratio reaches 50 ppbv in the mid-troposphere in November. South of the source region, low ozone mixing ratios of 20--25 ppbv are calculated in the boundary layer due to marine air influence and reduced photochemical activity in presence of biomass burning aerosols. The particular transport regime prevailing during the 1997 El Ni¿o event is not considered in our calculations. This limitation precludes any definitive conclusion regarding the relative role played by photochemistry and transport processes on the distribution of species during the 1997 fires. ¿ 1999 American Geophysical Union