Invasive American mink and native polecats were live-trapped over a period of six years and radio-tracked during one winter-spring season in the lakeside habitats in NE Poland. The number of mink declined whereas number of polecats was stable during 1995–2000, however, except during one winter, mink were always more abundant in the study area than polecats. Significant differences in habitat utilization between radio-collared mink and polecats were observed. Mink moved only along the lake shoreline and showed no seasonal shift in habitat selection. In winter, polecats were most frequently located close to the lake banks, but they also stayed in barns and stables. In spring, they moved further from the lakes. There was considerable interspecies overlap of mink and polecat home ranges in February, and common use of the banks of the 500 m long unfrozen canal was recorded for 4 mink and 5 polecats. The pattern of daily activity of polecat and mink differed: mink were most active at dawn and in early morning whereas polecats at dusk and in beginning of the night. Individuals of both species coexisted in this small area at relatively high densities and to some degree exploited the same habitats, particularly in the vicinity of sites with access to open water.