Immature individuals were taken in October, November, February, and March, and a female with two large embryos was taken near Icehouse Canyon on November 8, 1951. _Specimens examined._--Total, 8, distributed as follows: Los Angeles County: Mescal Wash, 4000 ft., 1; Mescal Canyon, 4800 ft., 2; San Antonio Canyon, 5200 ft., 2; San Antonio Canyon, 4500 ft., 1; San Antonio Canyon, 2800 ft., 1; Thompson Canyon, 1800 ft., 1 (PC). =Peromyscus truei montipinoris= Elliot Piñon Mouse Only once was this mouse found outside the pinyon-juniper association of the desert slope; in November, 1949, several were collected near Cajon in mixed manzanita, scrub oak, and greasewood chaparral. This was the only _Peromyscus_ of regular occurrence in the pinyon-juniper area, and was recorded from the upper limit of this association, near Jackson Lake, at 6000 feet, to the lower limit of the association at the mouth of Graham Canyon at roughly 4000 feet elevation. Although in the juniper belt _truei_ often occurs on common ground with _Peromyscus maniculatus sonoriensis_, the habitat preferences of these animals are generally complementary. Where the mice occur together, traps set in a variety of locations caught _Peromyscus maniculatus_, but typically traps set amid the brush or on the open ground away from the junipers were productive. On the contrary _truei_ was invariably trapped quite near the junipers and often in association with the large nests of _Neotoma fuscipes simplex_. In fact traps set right on the beds of litter beneath the junipers were most likely to catch _truei_. Records kept of trapping localities show that _truei_ was without exception trapped within twenty feet of some treelike shelter such as junipers, pinyons, Joshua tree or scrub oaks. Thus _Peromyscus maniculatus_ occupies the open stretches between the trees, while _truei_ inhabits the ground beneath and immediately adjacent to the trees. In Nevada the piñon mouse prefers rocky areas (Hall, 1946:520). In the San Gabriel Mountains this mouse does not seem to have this predilection. In the juniper belt _truei_ was second to _Dipodomys panamintinus_ in point of numbers. In the course of 500 trap-nights in the juniper belt twenty-two _truei_ were taken with thirty-six _Dipodomys_.