Lamb predation in Patagonia, Argentina, is considered by ranchers as the cause of one of their greatest losses and limits sheep production. Patagonian red fox (Dusicyon culpaeus) is the main predator and the magnitude of the problem reflects its distribution and abundance. Since 1979, the Bariloche Experimental Station of the Instituto Nacional de Technologia Agropecuaria (INTA) has performed 8 studies, totaling 1,717 lamb necropsies, with the purpose of determining the relative importance of predation and other causes of lamb mortality. This paper analyzes the causes of lamb mortality, with special reference to red fox and carnivorous bird predation. Red fox predation pattern is also discussed, based on the analysis of 157 cases of red fox kills.