In Washington _Peromyscus_ may be separated from other naked-eared mice by ungrooved teeth and slightly tapering, bicolor tail. Nearly every part of North America is inhabited by one or more kinds of _Peromyscus_. The single species, _maniculatus_, which occurs in Washington, ranges from the Atlantic to the Pacific and from the Arctic to tropical Mexico. Osgood (1909: 17) remarks: "... it is probable that a line, or several lines, could be drawn from Labrador to Alaska and thence to southern Mexico throughout which not a single square mile is not inhabited by some form of this species." Deer mice are the most abundant mice in Washington. They occur at sea level on the ocean beaches and above timber-line, even breeding in the Arctic-alpine Life-zone. They occur on the islands in Puget Sound, the lowland marshes and deciduous thickets of western Washington, the great conifer forests, the alpine cirques and mountain parks, and the deserts of eastern Washington. Almost everywhere they are the commonest mammal encountered. They make their homes in city dwellings, under logs or in hollow trees in the forest, in the burrows of larger rodents and, if necessary, in burrows of their own construction. Nests are usually cup-shaped masses of soft material, 4 to 8 inches in diameter with central cavities 2 to 3 inches in diameter. Materials utilized include dry grass, shredded bark, rope, rags, newspaper, moss, cattail fluff, fur, wool, and feathers. One nest discovered in the attic of an old building near Coulee City, Grant County, consisted of the mummified body of a wood rat (_Neotoma cinerea_). The fur of the arched body of the rat formed one side of the nest, while the fur of the far side of the body had been plucked to form the remainder. In the extensive marshes along Lake Washington, King County, deer mice occupy the ball-shaped nests of tule wrens (_Telamtodytes palustris_). These are constructed entirely of fragments of cattail leaves and cattail fluff and are placed in the cattail rushes about four feet above the wet ground of the marsh. Other deer mouse nests were found in an unused typewriter, in the pocket of an old coat, in a window frame, under a piece of tar paper on the ground, in a cavity in the ground under a board, between a ceiling and a loose bit of roofing paper, under rocks and logs, in hollows in logs, and in an unused nest of a wood rat. Where plant cover is dense, nests are placed on the surface of the ground, as under thick growths of tumbleweed (_Salsola_) in eastern Washington.