_Distribution._--The Columbian Plateau, the Upper Sonoran Life-zone area west of the Columbia River and south of the Wenatchee Mountains in Kittitas, Yakima, and Benton counties, the north side of the Columbia River in Klickitat County, and Okanogan County east of the Okanogan River. Marginal localities are: Timentwa (W. W. D.) on the north, Maryhill (M. V. Z.) on the west, Wallula (M. V. Z.) on the south, and Colfax (Taylor and Shaw, 1929: 21) on the east. [Illustration: FIG. 105. Distribution of the western harvest mouse, _Reithrodontomys megalotis megalotis_, in Washington.] _Remarks._--Harvest mice from the states of Washington, Oregon, Idaho, Utah, Nevada, California, Arizona, and New Mexico were examined to ascertain the validity of the name _Reithrodontomys megalotis nigrescens_ Howell (type from Payette County, Idaho). No cranial characters or measurements could be found to separate _nigrescens_ from _megalotis_. Specimens from Washington, Oregon, and west-central Idaho, within the range ascribed to _nigrescens_, averaged slightly darker in color with a more distinct, blackish dorsal line than specimens from New Mexico. This difference is slight, and specimens from any one locality vary greatly in color. Some Washington specimens are as pale as the palest material from New Mexico, and some of the darkest _nigrescens_ can be matched by specimens of _megalotis_ from Arizona and New Mexico. This slight color variant is not worthy of recognition as a distinct subspecies, and I agree with Howell (1939: 390) that _nigrescens_ is a synonym of _megalotis_. _Description._--The western harvest mouse is about the size and shape of the common house mouse. The length of the tail is approximately equal to the length of the head and body. The tail is slim and lightly haired. The upper parts are brownish buff lightly washed with blackish. A faint but distinct dark dorsal stripe is usually present. The underparts are white. From _Mus_ and _Peromyscus_, _Reithrodontomys_ can be distinguished by the grooves on the anterior face of the upper incisors and the long brown hairs at the bases of the ears.