Since January 1994, continuous surface O3 measurements have been carried out at Amsterdam Island (37 ¿S, 77 ¿E) in the southern Indian Ocean using a UV absorption analyzer. Mean O3 concentrations and standard deviations are recorded every 5 min. This paper shows and discusses the O3 time series for a 3-year period (1994--1996). During this period, O3 shows a seasonal variation with maxima around 30 ppbv during winter (July--September) and minima around 13 ppbv during summer (December--February). O3 levels at Amsterdam Island are close to those observed at Cape Grim (Tasmania, 41 ¿S, 145 ¿E) for the period of 1991 to 1995. In order to improve the understanding of the mechanisms controlling the O3 seasonal variation at Amsterdam Island, our observations have been analyzed by using the global three-dimensional climatological Model of the Global Universal Tracer Transport in the Atmosphere (MOGUNTIA). The model reproduces well the observed O3 mixing ratios and their seasonal cycle. The seasonal variation of stratospheric origin O3 calculated by MOGUNTIA shows a maximum of 18 ppbv in September and a minimum of 13 ppbv in April. The oxidation of continentally emitted O3 precursors during their transport to Amsterdam Island contributes to the photochemical production of O3 by up to 12 ppbv during austral summer. From this amount, 3--7 ppbv are directly linked to nonmethane hydrocarbon oxidation chemistry under relatively high NOx conditions. In particular, biomass burning emissions contribute up to 5.5 ppbv to the observed O3 levels in September. Both photochemical production by biomass burning and stratospheric influx of O3 lead to a maximum in O3 mixing ratios during late winter to early spring in agreement with the observations. Minimum O3 mixing ratios observed during austral summer are related to photochemical O3 depletion. ¿ 1998 American Geophysical Union