The red-eared slider (Trachemys scripta elegans) is a worldwide highly invasive species, currently introduced in most freshwater habitats as a consequence of massive pet trade. In the Iberian Peninsula, this species is competing with and displacing the endangered native Spanish terrapin (Mauremys leprosa). Sliders are considered environmentally aggressive turtles, capable of threatening or biting other individuals during competitive activities such as feeding. We hypothesized that agonistic behavior of introduced sliders against native terrapins might negatively affect the feeding efficiency of M. leprosa. We compared food ingestion of turtles and aggressive interactions during feeding, under situations of conspecific and heterospecific competition. The amount of food ingested by native and introduced turtles was similar under conspecific competition, but T. scripta ingested a greater percentage of food supplied under heterospecific competition.