Rainbow trout (Oncorhynchus mykiss) have been widely introduced outside of their natural range and are one of the World's most invasive species. Illegally introduced into the isolated, high-altitude (4520 m) Chungar & aacute; catchment of northern Chile, rainbow trout have now been present in the system for similar to 30 years and to our knowledge are the World's highest known self-sustaining population. However, nothing is known about the status and impacts of these trout in the system. We analysed stomach contents and stable isotopes (delta 13C, delta 15N, delta 34S) from liver and muscle to study the trophic ecology of rainbow trout from lake, river mouth and riverine habitats in the Chungar & aacute; catchment to understand their basic ecology and to characterise their potential trophic interactions with native taxa. Both stomach contents and stable isotope values showed spatial differences in diet and habitat use, but also revealed that diet varied temporally within habitats. The trophic position of rainbow trout was similar in the three habitats examined and was constant over time, apart from the riverine population. Predation by rainbow trout had different potential effects across pelagic, benthic, and riverine habitats in the Chungar & aacute; catchment, through the consumption of zooplankton, native amphipods, aquatic insects, and the endemic cypriniodontid fish Orestias chungarensis.