Ten short-tailed weasels (Mustela erminea bangsi), in white pelage, were trapped in Taylor County, Wisconsin during November and December 1960. On 11 January 1961, the animals were divided into 2 groups of 5 animals each and placed in environmental rooms having artificial daylengths of 18 hours. Group I was exposed to a constant temperature of ca. 70°F; Group II was exposed to a constant temperature of ca. 20°F. All of the animals had molted into a brown pelage by 16 March 1961. The onset of the molt was delayed in the cold room; but once started, proceeded rapidly, the white winter pelage being completely replaced by brown as the molt progressed. As white hairs were shed in the warm room, brown hairs appeared, producing intermixed areas of brown and white.