ABSTRACT Geranoaetus polyosoma (Quoy & Gaimard, 1824) is a diurnal raptor widely distributed in South America. Although the trophic ecology of this bird has been more studied in the southern extreme of its range, little information is available on its dietary response to prey supply in desert environments. In the present study, we report on the trophic ecology of G. polyosoma in a sub-urban desert zone in northern Chile, with the following objectives: (1) to quantitatively describe its diet and (2) to determine its dietary selectivity in response to prey supply in the study area. The diet of G. polyosoma consisted mainly of rodents (97.2%). A greater preference (p < 0.05) was observed for the following large prey items (> 19.5 g): two native rodent species, Phyllotis xanthopygus (Waterhouse, 1837) and Eligmodontia puerulus (Philippi, 1896); and two introduced rodent species: Rattus rattus (Linnaeus, 1769) and R. norvegicus (Berkenhout, 1769).