This species occurs from 1000 feet elevation to above 9000 feet elevation on the Pacific slope of the Mountains, but although probably the most widespread rodent in the area it is absent from many habitats. This mouse reaches maximum abundance in the coastal sage scrub association, particularly where the soil is sandy with scattered vegetation--usually coastal sagebrush and black sage. On the foothill adobe slopes none was trapped, nor have any been taken in most of the chaparral habitats. A few _gambeli_ were trapped amid the talus beneath growths of scrub oak and bay trees in San Antonio Canyon, at 4300 feet elevation. On Blue Ridge, at elevations of from 7200 feet to 8300 feet, this mouse inhabited areas clothed with snowbush, basin sagebrush, currant, and scattered conifers, and was found sparingly in the coniferous forests. Thus this species lives on contrasting soil types in association with many different vegetational assemblages, from the coastal base to the crest of the range. There is a rather wide variation in color in _gambeli_ from the San Gabriels. Certain individuals taken in open, sandy coastal sage areas are pale, some being indistinguishable from examples of _sonoriensis_ taken in the pinyon-juniper association on the desert slope. Specimens from San Antonio Canyon have somewhat darker pelage than those from the sage belt, and than individuals taken on Blue Ridge. Possibly a large series of _Peromyscus maniculatus_ from the San Gabriel Mountains would show definite local trends in color of pelage. This species is active on sub-freezing and rainy nights as evidenced by trapping results, and at Big Pines there were tracks around the bases of conifers after a heavy snowfall in December, 1951. Several females taken in the sage belt in October, 1948, carried embryos, and a lactating female was recorded from Blue Ridge on November 13, 1951. Juveniles have been taken in September, October, November, and December. _Specimens examined._--Total, 9, distributed as follows: Los Angeles County: 1 mi. S and 2 mi. W Big Pines, 7400 ft., 3; 1 mi. S and 2 mi. E Big Pines, 8200 ft., 1; 4 mi. NE Claremont, 1900 ft., 2; San Antonio Wash, 1800 ft., 3 (PC). =Peromyscus maniculatus sonoriensis= (Le Conte) Deer Mouse This subspecies is associated with contrasting types of soil and vegetation. It is seemingly absent from the upper pinyon-juniper sage flats and areas grown to chaparral, but is fairly common on the gravelly benches dotted with junipers, and in the washes issuing from the canyons on the desert slope. It is present in small numbers in the Joshua tree association.