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Diet of the Magellanic Horned Owl (Bubo magellanicus) in Andean localities of northwestern Argentina. - We studied the diet of the Magellanic Horned Owl (Bubo magellanicus) in three Andean localities of northwestern Argentina. From east to west, localities were placed in three different environmental contexts: high altitudinal grasslands of Yungas (Valle Encantado), Monte de Sierras y Bolsones (La Poma) and Puna (Las Cuevas). We identified 1532 prey-items, mostly small mammals with weights < 500 g, and secondarily birds and arthropods. Leaf-eared mice of the genus Phyllotis, a nocturnal sigmodontine rodent typical of rocky areas, were the most predated item by owls. Previous studies coincided in emphasizing the importance of leaf-eared mice as prey of owls in Andean areas of Argentina and Chile. In all localities, one or two prey contributed with more than 50% of the biomass to the diet. Values of standardized food niche breadth (FNB) varied between 0.07 and 0.33, while the mean weight of prey varied between 40.9 and 49.1 g. The FNB and the standardized FNB varied inversely with altitude, while mean prey weight did not change along the gradient. Our results are consistent with reports in literature that indicate that the Magellanic Horned Owl is a predator specialized in the consumption of small mammals. However, to achieve a comprehensive understanding of trophic ecology of this owl in high Andean areas further studies are needed to assess the seasonal fluctuations of the prey and the relationship between these variations and their actual abundance in the field.