The spatial organization of a species on a landscape is influenced, at least in part, by the presence of sympatric competitors. Interspecific relationships can thus have direct effects on the carrying capacity of the landscape and have important implications for conservation. We investigated the spatial relationships and activity patterns of 2 similarly sized mustelids: the invasive American mink (Neovison vison) and the native European polecat (Mustela putorius) in lowlands in the United Kingdom. By radiotracking mink (n = 11) and polecats (n = 7) in autumn when individuals of both species hold stable home ranges, we found that individuals tended to have overlapping home ranges, both within and between species; and the size of overlap areas was similar, but generally small, within and between species (mean approximately 20%, although overlaps were higher among mink of opposite sexes). Mink shared their home ranges with 0.3–1.17 other mink (of both sexes) and 0.83–1.3 polecats; polecats shared their home ranges with 0.6–1.0 polecats (of the opposite sex) and 1.6–2.0 mink. Neighbors avoided simultaneous use of overlap areas; polecats were nocturnal and mink were predominantly diurnal. Our results are consistent with interspecific territoriality although we cannot distinguish between interspecific territoriality and niche differentiation. We suggest that there is habitat partitioning between the 2 species, but that this is incomplete and that temporal partitioning enables avoidance of interspecific neighbors within overlap areas. Niche partitioning by distance from water and time of day when active probably facilitates coexistence in the short term, but it is unclear whether coexistence is stable year-round or in the long term.