The long-tailed weasel is larger than the ermine, but they are similar in appearance. Its tail is more than 44% of its total body length and the black tip on the tail is relatively longer than the ermine's. During summer the long-tail weasel is light to dark brown dorsally, and its belly is buff to rusty orange; its chin is white. In winter it turns white except for the black-tipped tail. Males are up to 40% larger than females.
This species is found throughout North America south of southern Canada except for the extreme Southwestern part of the U.S.
Long-tail weasels inhabit a variety of habitats from open woods to grasslands, and river bottoms to fencerows. In Idaho, occurs from upland brush and woods to subalpine rock slides and semi-open forest areas, but is most numerous in rocky, mountainous regions. Generally, it is found in more open habitat than the ermine.
This weasel feeds primarily on small mammals (pocket gophers, mice, ground and tree squirrels and chipmunks), and occasionally on birds and their eggs, other small vertebrates such as snakes and frogs, and insects. Being larger than females, males prey on larger prey, up to snowshoe hare in size. When prey is abundant they will make multiple kills and cache their prey.
Activity of the long-tailed weasel is primarily nocturnal, but they are frequently seen during daytime. When inactive, they occupy rock crevice, brushpiles, hollow stumps, space among tree roots, or abandoned burrows made by other mammals; one individual may use multiple dens. Their home range is larger than the ermine, 80 to 120 ha (200 to 300 acres), but their movement through their habitat is variable, depending on the availability of prey. Individuals are basically solitary. population density depends on habitat and environmental conditions, and averages 1 weasel per 3 -16 ha.
Like the ermine, the long-tailed weasel has delayed implantation. Breeding is in July and August when both juvenile and adult females come into estrus. After a short period of development, the fertilized egg is quiescent until implantation in the uterus in early spring. The tiny young are born in April and May. They develop juvenile teeth in 3 to 4 weeks, when they can probably feed on some meat supplied by the mother. By 6 weeks they are weaned, and they disperse from the mother in 10 to 12 weeks.