The feeding ecology of the Patagonia puma (Felis concolor patagonica) was studied in Torres del Pain, National Park, Chile, by analyzing seasonal and yearly variation in 590 prey items found in 405 puma feces from 1982 to 1988. Mammalian species accounted for 92% of all prey items in feces. European hares (Lepus capensis) were the most preyed-upon vertebrate, representing 50% of the items found in feces. Guanacos (Lama guanicoe) made up 32% of prey items and accounted for 47% of the total biomass consumed by pumas. From 1982 to 1988, the proportion of guanaco remains in puma feces increased from 9% to almost 30% of total prey items, paralleling an increase in the guanaco population from 670 to 1,300 individuals in the study area. The only livestock species found in the puma diet was sheep (Ovis aries), representing 5 % of the total number of prey found in feces. Small- and medium-sized prey may be important alternative prey in areas where large prey are less abundant.