Populations of feral American mink Mustela vison were investigated in several study areas in Ireland, principally by capture-recapture techniques. The populations were found to be essentially self-regulating, with stress, weight loss and mortality evident during the winter and spring. The highest mean density recorded, in terms of animals per unit length of river or lake shore, was 1.4 individuals km−1 with a peak density of 2.3 individuals km−1, but population densities were considerably lower in most areas. Trappability and population structure, in age, sex and residency classes, varied between habitats, but population turnover was high, leading to complete replacement of individuals in two populations monitored for two years. The availability of the preferred prey, crayfish Austropotamobius pallipes appeared to be a major factor determining population size and preferred breeding areas. Control methods are either unnecessary or are likely to fail in most situations, and may be counterproductive.