We determined the diet of a small population of Huemul deer ( Hippocamelus bisulcus ) in Nahuel Huapi National Park, Argentina, using microhistological analysis of fecal samples. The study area is of special interest for conservation because of the ingression of the introduced European red deer ( Cervus elaphus ). Thirty-two food items were identified as components of the diet, with Maytenus sp. and Nothofagus pumilio being the most consumed. These items also comprise a high proportion of the diet of red deer, which leads us to expect high trophic overlap wherever these two herbivores coexist, producing the displacement of the huemul to less favorable areas, and increasing its vulnerability.