Body sizes of European mink (Mustela lutreola L.), polecat (M. putorius L.) and American mink (M. vison Schreber) were studied over a 10-year period in an area of north-eastern Belarus, before and after the invasion by American mink, and data are presented on interspecific interactions. On arrival in the study area American mink males were larger than males of European mink and polecat, and American mink females were larger than females of the other species. After arrival of the American mink its mean body size decreased, whilst the resident male and female European mink and female polecat increased as measured in absolute mass, length and relative mass. The observations suggest a strong character convergence most plausibly explained as a response to the invading exotic by the residents as well as in the invading species itself, whilst a divergence had been expected. There was no evidence to show whether these differences were genetically based. The body size data are consistent with the hypothesis that European mink, and to a lesser extent polecat, are responding to direct aggression from American mink (rather than merely competing for resources), with the smaller individual European mink being more likely to disappear first. We provide direct evidence for the aggressive nature of inter-specific relations from observations using radio-tracking: all observed inter-specific interactions were aggressive, significantly more so than intraspecific encounters, causing the European mink to flee, and several left the study area altogether. Implications for niche theory and for conservation management are discussed.