On the desert slope of the mountains this species is found in the part of the pinyon-juniper association that is between elevations of 4000 and 5200 feet. The mouse is absent from the higher chaparral and pinyon-covered slopes, but is present on south slopes in the pinyon belt where more open growths of pinyons and scrub oaks are interspersed with yucca. I recorded this pocket mouse from the vicinity of Cajon Pass west to Valyermo. The local distribution of _pallidus_ is striking because of its close positive correlation with the distribution of yucca. On benches around 5000 feet, where yuccas are scattered in their occurrence, _pallidus_ is nearly always taken near (often right at the base of) this plant. Lower in the juniper belt the dry rocky south slopes supporting yucca plants are well populated by _pallidus_, while adjacent flats, and north slopes grown to antelope brush and scrub oak, are completely uninhabited. Near the mouth of Grandview Canyon, on steep rocky southern exposures grown sparsely to burro weed and yucca, one hundred traps produced in one night eight _pallidus_ and no other rodents. Here many of these pocket mice were trapped on large fractured rock outcroppings, where most or all of the mice probably lived in the daytime in the deep cracks; in any event no burrows were noted near these rocks. This species prefers barren slopes supporting yucca plants. These plants produce large seeds which are staple food items for _P. f. pallidus_ and other rodents during the lean part of the year, that is to say, late summer and autumn. Many of the dry capsules of the yucca plants were examined in October, 1951, and these generally still contained a few seeds. Pocket mice taken in October usually carried in their cheek pouches seeds of yucca together with some other material, and often they carried only the seeds of yucca. Probably the wind shakes only a few seeds out of the capsules at a time, thus tending to drop the seeds over a fairly long period. Trapping in winter in the juniper belt revealed that these pocket mice were not active above ground on nights colder than about 40° F. On nights when the temperature was about 36° F. none was taken, but on the one night in late December, 1948, when the minimum was 44° F., several specimens were taken. In this same area in May 1949, pocket mice were the most numerous rodents. Because of their evident sensitivity to cold weather, these mice must remain below ground for weeks at a time during the cold weather of December and January.