Grasshopper mice of two species range over much of northern Mexico and the western United States. The species occurring in Washington, _leucogaster_, is found from southern Manitoba to northern Mexico, and from the Pacific states east to North Dakota. This species is characteristic of the Upper Sonoran Life-zone, and usually is associated with sagebrush and greasewood. It prefers open areas of sand and avoids heavy cover, and seems to be strictly nocturnal. It eats vegetation, seeds, insects, or the flesh of other mice. According to Bailey (1936: 178), grasshopper mice pursue, catch and kill other mice. They hunt by scent and follow tracks like a wolf. They make a sound, or "call," like a miniature wolf howl. They are said to be rather docile and make friends with humans quickly. On some occasions grasshopper mice are locally abundant, but for the most part they are rather uncommon over their range. Usually one is taken in about 200 traps set in suitable habitats. They do not hibernate; one was caught at Moses Lake, Grant County, on a freezing winter night. The grasshopper mouse is almost always associated with the pocket mouse (_Perognathus parvus_), deer-mouse (_Peromyscus maniculatus_), and often the harvest mouse (_Reithrodontomys megalotis_). The same snakes, owls, and carnivorous mammals that prey on these mice doubtless eat also the grasshopper mouse. R. D. Svihla (1936: 172) determined the gestation period of 3 lactating grasshopper mice captured at Lind, Adams County, as 33, 39 and 47 days. A non-lactating female gave birth to a litter in 32 days. Young varied from 2 to 5 in number and were pink and hairless at birth, with eyes and ears closed. =Reithrodontomys megalotis megalotis= (Baird) Western harvest mouse _Reithrodon megalotis_ Baird, Mamm. N. Amer., p. 451, 1857. _Reithrodontomys megalotis nigrescens_ Howell, N. Amer. Fauna, 36:32, June 5, 1914 (type from Payette, Idaho). _Reithrodontomys megalotis megalotis_ Bailey, N. Amer. Fauna, 55:189, August 29, 1936. _Type._--Obtained between Janos, Chihuahua, and San Luis Springs, New Mexico, by C. B. R. Kennerly, in 1855; type in United States National Museum. _Measurements._--Five males and 6 females, showing moderate wear on the third upper molars, from Walla Walla County, average, respectively: total length 141, 137.5; length of tail 71.8, 68.1; hind foot 17.2, 16.7; weight (males only) 10.5 grams. A female, 136 mm. long, from Grant County, weighed 9.5 grams. Over the range of _megalotis_ in the United States, males average consistently larger than females in length of the hind foot and consistently less in length of head and body.