Abstract The development of mixed grazing systems is an interesting alternative for the utilization of rangelands with ecological and/or economic limitations; therefore, the study of patterns and eating habits of different species in the same grazing area is important. A mixed grazing study was carried out on a Mediterranean grassland sown with Thinopyrum ponticum 30 years ago (Rinconada de Maipú, Chile 33° 28’S; 70° 51ʼ W) with the purpose of quantifying the botanical composition, dietary overlap and selectivity indexes of the main grass species consumed by goats and sheep. Trophic behavior was studied in seven young Suffolk Down rams, seven young Merino-Precoz rams and seven young Boer-Criollo bucks grazing together during three grassland phenological stages. During the grassland vegetative stage, diets were mainly composed of annual grasses and forbs. Perennial grasses dominated the animals’ diets during the reproductive and dry grassland stages. The diversity of the goats’ diets was higher than that of the sheep, demonstrating greater plasticity in their dietary habits. However, in this environment, both species behaved as generalist herbivores. The level of diet overlap between the sheep and goats was high, especially when grazing during the grassland reproductive and dry phenological stages, suggesting potential competition during lower forage quality stages.