The red-backed mice, including more than one species, range over the boreal parts of the Old and New World. In America they are found in most of Alaska, Canada and the northern United States, and extend southward in the Rocky Mountains and along the Pacific Coast. The one species _Clethrionomys gapperi_ ranges across southern Canada from the Atlantic to the Pacific, extending southward in forested areas to New Mexico. They usually live in clearings in the forest. In the Pend Oreille Mountains _saturatus_ was common in damp, mossy talus slides, along with _Microtus longicaudus_ and _Phenacomys intermedius_. The _Clethrionomys_ outnumbered the two latter species combined by 25 to 1. In nearby forest, red-backed mice were scarce but no other microtines were found. In the Kettle River Mountains a week later, red-backed mice were rare. A single specimen was taken in a damp place in the forest; none was found in talus slides. Near Stevens Pass, King County, in the Cascades, _cascadensis_ was taken in equal numbers in talus slides and under logs in the forest. Near Dewey Lake, Yakima County, in the Cascades, I took them only in an extensive grassy meadow. In the Blue Mountains I found _idahoensis_ in the dense chaparral, far from forests. Unlike _Microtus_, red-backed mice do not make runways, although they sometimes follow the runways of other mammals. Taylor (1920B: 92) found red-backed mice breeding on Mount Rainier from early July to the middle of September. One female gave birth to four young in a nest in his duffle bag. [Illustration: FIG. 111. Distribution of the Gapper and California red-backed mice in Washington. A. _Clethrionomys gapperi saturatus._ B. _Clethrionomys gapperi idahoensis._ C. _Clethrionomys gapperi nivarius._ D. _Clethrionomys gapperi cascadensis._ E. _Clethrionomys californicus occidentalis._] =Clethrionomys gapperi saturatus= (Rhoads) _Evotomys gapperi saturatus_ Rhoads, Proc. Acad. Nat. Sci. Philadelphia, p. 284, October 23, 1894. [_Clethrionomys gapperi_] _saturatus_ Whitlow and Hall, Univ. California Publ. Zoöl., 40:265, September 30, 1933. _Type._--Obtained at Nelson, British Columbia, by S. N. Rhoads on August 27, 1892; type in Philadelphia Academy of Sciences. _Racial characters._--Size small, about 140 mm. in total length; tail short, about one-third of total length; ears large, projecting above fur; color of head and sides gray tinged with yellowish; back with distinct red stripe; underparts whitish tinged with buff. _Measurements._--Five males and 10 females from Pass Creek Pass, Pend Oreille County, average: total length 147; length of tail 43; hind foot 18.3; ear 14. _Distribution._--Northeastern Washington from Sherman Creek Pass (W. W. D.) on the west to Pass Creek Pass on the east. =Clethrionomys gapperi idahoensis= (Merriam)