In 1951 the numbers of _sonoriensis_ were noticeably less than in 1948; probably this was correlated with the series of dry winters in this period. In December, 1948, this animal was one of the most common rodents in Mescal Wash, 200 trap-nights yielding thirteen specimens; but in November, 1951, none was taken. In parts of the juniper belt, where an average of about six _sonoriensis_ was taken per 100 trap-nights in 1948, the average had dropped to one per 100 trap-nights in 1951. Specimens of this species from the desert slope of the mountains have been assigned to the subspecies _sonoriensis_. Those from Blue Ridge tend toward _sonoriensis_ in color, and may be considered as intergrades between this subspecies and _gambeli_. This species was active on nights when the temperature was as low as 10° F., and individuals were trapped in the juniper belt in December, 1948, when four inches of snow lay on the ground. Gray-pelaged juveniles were taken on the desert slope in December, 1948, and a female taken in Mescal Canyon on December 22 of this year carried four embryos near term. _Specimens examined._--Total, 11, distributed as follows: Los Angeles County: 8 mi. E and 4 mi. S Llano, 4000 ft., 6 (4 PC); Mescal Canyon, 4800 ft., 5. =Peromyscus boylii rowleyi= (J. A. Allen) Brush Mouse The main range of this mouse in the San Gabriel Mountains lies between 1600 and 6000 feet elevation on the Pacific slope of the Mountains, thus encompassing much of the chaparral and oak woodland associations. It was the most common mammal in the oak woodland association in the lower foothills and often was trapped there on leaf mold beneath the oaks. While trapping for shrews I regularly took this species in riparian growth right down to the edge of the water. In San Antonio Canyon many _boylii_ were trapped beneath logs and dense vegetation, and on wet seepage slopes adjacent to the creek. This species shows a definite predilection for rocky habitats where these occur in the chaparral. In heavy lilac brush near Camp Baldy _Peromyscus boylii_ was outnumbered by _P. californicus_, yet where talus slopes or boulder piles occurred _boylii_ was more numerous. At the head of Cow Canyon amid boulders beneath scrub oak, bay, and big cone-spruce, this species was especially abundant and no other _Peromyscus_ was taken. Of special interest is the occurrence of this mouse on the desert slope of the mountains; there it was taken beneath scrub oaks in the pinyon-juniper association at the mouth of Mescal Canyon, and amid boulder and debris piles in Mescal Wash at 4000 feet elevation. While manzanita and scrub oak grew in the wash at the points of capture, the animals were actually surrounded by the desert conditions of the Joshua woodland, and associated with such desert forms as _Onychomys torridus pulcher_ and _Peromyscus eremicus eremicus_.