South America is one of six global "hotspots" of invasion of freshwater fish, and Chile is no exception. There are currently 27 species of introduced fish in Chilean freshwater ecosystems. Twenty five of these 27 exotic species belong to families that did not exist in Chile before; fish from families such as Salmonidae, Cyprinidae and Ictaluridae are the most abundant and frequent. Thus, native Chilean fish (45 species) are facing competitors and predators that are unknown to them in evolutionary terms. In this paper, we analyse the invasion of salmonids in Patagonia. These salmonids, after a century of introductions, seedlings and escapes, are often the most common fish in Patagonian rivers and lakes and present a wide latitudinal and altitudinal distribution. We describe the effects of salmonids on native fish (mainly galaxiids) and analyse the mechanisms by which these effects occur. We conclude that native fish species are "naïve" and seem to lack physical abilities and behavioural attitudes to counter or avoid negative interference of salmonids. Therefore, it is urgent to improve current management measures associated with introduced species in Chile in order to conserve native freshwater biodiversity.