To better understand the climate, vegetation and fire history of southern South America since the last deglaciation, sediment characteristics, diatoms, pollen, microscopic and macroscopic charcoal and magnetic susceptibility were analyzed in a 14-m long sediment core from a closed basin fen at Lago Pollux, Aisén Province, southern Chile (45°41'30" S lat, 71°50'30" W long, 640 m elev.). Sediment characteristics, absence of diatoms and scarce pollen suggest that before 18,000 cal yr BP, glacier meltwaters were feeding the small basin when local glaciers were at their maximum position. From 18,000 to ca. 14,000 cal yr BP, the site was a shallow productive pond surrounded by a sparse scrub-steppe and accompanied some fire activity. This period was markedly drier, but warmer than before. By 14,000 cal yr BP, the fen developed and a species-rich steppe expanded suggesting a moderate increase in effective moisture and temperature. High fluctuations of terrestrial and aquatic pollen taxa occur between 14,000 and 11,000 cal yr BP, attesting to climate variability at this time. Between 11,000 and 7500 cal yr BP, Nothofagus steppe-woodland developed and fire activity was very high, with several large local events. Precipitation must have increased, but most likely summers were drier than today with high frequency of convective storms. After 7500 cal yr BP present-day closed Nothofagus forest became established, and fire activity decreased, suggesting establishment of present-day equable precipitation regime. The forest was more open between 4000 and 1500 cal yr BP than before and shrubs or trees grew on the fen. Forest clearance and grazing only began in the mid-20th century. Both in terms of timing and character of past changes in vegetation and fire activity, the record from M. Pollux takes an intermediate position between records from high southern latitudes, versus those from mid-latitudes, indicating the different latitudinal expression of the southern westerly storm tracks during the last 18,000 cal yr BP.