Live trapping and tracking were used to study ermine (Mustela erminea) ranges and populations in the Boyd Conservation Area, Kleinburg. Ontario. A total of 327 captures were made on 16 males and 11 females. Monthly population densities on the 95-ha site averaged 5.97 individuals per square kilometre overall and 10.53 individuals per square kilometre in preferred habitats. Despite the influx of new arrivals the population remained stable throughout the 22 months of investigation. Some individuals remained on the site in excess of a year, but most did not. Individual ranges tended to be spatially stable, with male ranges averaging 20–25 ha and those of females 10–15 ha. Ranges overlapped considerably. Resident ermines always established ranges which included substantial portions of early successional communities.Ermines utilized their ranges on a regular basis and males travelled significantly greater distances per unit time than did females. Distributional data support the hypothesis that ermines are polygamous. All resident females gave birth during 1974.