The interaction between native and introduced predators can be an important determinant of the success of introduced species and of the magnitude of their effects. In Europe, it has been shown that the American mink Neovison vison can be affected by native competitors; however, such evidence has not been found in South America. We studied the southern river otter Lontra provocax and the American mink at five marine sites, where they coexisted, and at one freshwater site, where only mink were present, in southern Chile. We used the signs of both species to study their habitat use and diet, and radio tracking to study their activity patterns. The results indicated that otters and minks tended to use different habitats in marine environments, the otter favouring littoral areas that are rocky and steep while the mink favours areas of gravel with a gently sloping intertidal zone. These differences were also reflected in their diets. At one of the coastal sites where the diet of the two species was similar, the activity pattern of minks was mostly diurnal, which is unusual. While differential habitat use may be the way through which the American mink is able to coexist with the southern river otter in coastal habitats of southern Chile, it is possible that otters are having an effect on individual minks by affecting their activity patterns, although more observations are needed to confirm this hypothesis.