This species inhabited grassy areas of the coastal sage belt, and reached maximum abundance on cleared land grown thickly to weeds and scattered brush. The mouse was only locally abundant--being scarce throughout much of the sage belt--but was found under contrasting conditions. In San Antonio Wash the species was taken among rocks and sparse weeds, at Palmer Canyon specimens were trapped on a barren ridge sparsely clothed with greasewood and white sage, and also one mile E of Big Pines in flats supporting basin sagebrush and a fairly dense growth of grasses. The western harvest mouse was recorded from 1500 feet elevation to 3200 feet on the Pacific slope, and at 6600 feet near Big Pines on the desert slope. Those specimens of harvest mice from near Big Pines may be grading toward the desert race _megalotis_; my series of specimens from this locality, however, is too small for clear indications on this point. Individuals in juvenal pelage were taken on November 26, 1951, near Devore. _Specimens examined._--Total, 6, distributed as follows: Los Angeles County: 1 mi. E Big Pines, 6600 ft., 2; Palmer Canyon, 2000 ft., 1; 4 mi. N Claremont, 1700 ft., 3 (PC). =Peromyscus eremicus eremicus= (Baird) Cactus Mouse In Mescal Wash on the desert slope of the San Gabriels, this mouse was one of the most abundant mammals and was the only rodent other than _Peromyscus maniculatus_ regularly trapped in the barren channels of washes. In Mescal Wash, at an altitude of 4000 feet, _eremicus_ occurred along with the chaparral-inhabiting _Peromyscus boylii_ and _Peromyscus californicus_. The two species last mentioned were associated with the occasional large patches of manzanita, antelope brush, and other brush of the wash, whereas _eremicus_ was trapped in the rocky and sandy channels among scattered bushes of scale-broom. No specimens of _eremicus_ were taken on the juniper-clad benches adjacent to the wash. _Specimens examined._--Los Angeles County: Mescal Wash, 4000 ft., 10 (4 PC). =Peromyscus eremicus fraterculus= (Miller) Cactus Mouse This mouse was recorded from 1900 feet elevation, one mile south of the mouth of San Antonio Canyon, to 3200 feet elevation in Cajon Canyon. This subspecies is characteristic of the sage belt and shows a strong preference for the rough rocky areas found in dry washes. Although in many areas the channels of the washes are immediately adjacent to sandy sagebrush-covered flats, _eremicus_ is not common in the latter areas. Rocks seem to be essential to _eremicus_, for sandy areas in the sageland which were devoid of rocks yielded only an occasional specimen. For example, 100 trap-nights in the main channel of San Antonio Wash yielded 23 _eremicus_ and only six other rodents; while in the sandy sage areas nearby 200 trap-nights yielded only one _eremicus_ and 32 other rodents.