During a period in the 1980s when both European mink Mustela lutreola and American mink M. vison were present in Estonia, their food was significantly different. European mink ate a greater proportion of fish and crustaceans, whereas American mink took relatively more mammals and frogs. This was probably related to a difference in habitat selection. After the disappearance of the European mink, the diet of the American mink in our main study area was similar to that of the European in the same area previously. Two alternative hypotheses are presented for the mechanisms which led to the replacement of European mink by the American species: (i) the two species have a different niche, and the American mink could replace the European mink after the latter had disappeared for unrelated reasons, or (ii) the American mink aggressively ousted the European mink, a process starting in the American mink's preferred habitat (slow flowing rivers). At present there are insufficient data to reject either of these scenarios.