Restricted to the central and northern Baja California Peninsula and southwestern California . Range is widespread in the valleys of western Riverside and southwestern San Bernardino counties extending northwest to the vicinity of Claremont in Los Angeles County [2;3; both cited in 4]. Also found throughout western Baja California south to extreme northwestern Baja California Sur [2;5; both cited in 4]. Elevational range is from sea level to 1350 m (4500 ft) (Santa Rosa mountains, Riverside county) and 1800 m (6000 ft) (Cactus Flat, north slope of San Bernardino mountains) .
Known Populations in San Diego County
Within the MSPA, occurrences have been found in MU 1 (Sunset Cliffs), MU 3 (Crestridge), MU 4 (Mission Trails Regional Park, Sycamore Canyon and Goodan Ranch, Cleveland national Forest), MU 7 (TorreyPines), MU 6 (Elfin Forest Recreational Reserve, Santa Fe Valley), and MU 11 (Cleveland National Forest).
Common resident of sandy herbaceous areas, usually in association with rocks or coarse gravel [7;8; both cited in 6] in southwestern California. Occurs mainly in arid coastal and desert borders . Habitats tend to be stony soils above sandy desert fans and rocky areas within shrub communities such as coastal sage scrub, chamise-redshank chaparral, mixed chaparral, sagebrush, desert wash, desert scrub, desert succulent scrub, pinyon-juniper, and annual grassland [9, cited in 1;6].
Taxonomy and genetics
C. fallax has 6 recognized species  with two subspecies found in southern California: C. f. fallax from southern California through the coastal sage zone into the northern part of the Baja California Peninsula and C.f. pallidus in the eastern San Bernardino Mountains in California [1;4;10]. Formerly recognized as a subgenus of Perognathus before elevated to full generic stature 11, cited in 12].
Nocturnal [6;13]. Solitary . Active year-round  though avoids high daytime temperatures in its burrows [8, cited in 6].
Life history/ reproduction
Breeding occurs chiefly from March to May . Females can produce 1-3 litters per year [15, cited in 13]. An average of 4 young comprise a litter. Gestation is around 24-26 days . Young become sexually mature at 5 to 6 months of age. Typical longevity in nature is only 4-6 months, but it is not unusual for some individuals to survive 1-2 years [15, cited in 13].
Diet and foraging
Forages on seeds of forbs, grasses, shrubs with a low to moderate preference for forb and shrub seeds, and a high preference for grass seeds . Seeds transported within cheek pouches where it is stored in and around its burrow. May feed on some insects. Water obtained metabolically .
Home range varied from 0.19 to 0.45 ha, averaging 0.3 ha [17, cited in 10] No difference between sexes in home range size, and little overlap between sexes in home ranges, possibly the result of territoriality . Territory probably the same size as home range .
Threatened by habitat fragmentation and loss due to development .
Northwestern San Diego pocket mouse sources
 Rios, Evelyn, and Sergio Ticul Álvarez-Castañeda. 2010. Phylogeography and Systematics of the San Diego Pocket Mouse (Chaetodipus fallax). Journal of Mammalogy 91(2): 293-301.
 Hall, E. R. 1981. The Mammals of North America, 2nd edition. John Wiley & Sons, New York.
 Williams, F.N., Stace, C.A., Gornall, R.J., Squirrell, J. and Shi, Y., 1993. Taxonomy. In In: Genoways, HH, Brown, JH (Eds.), Biology of the Heteromyidae. Special Publications, American Society of Mammalogists.
 Erickson, R. A. and M. A. Patten. 1999. Identification and Distribution of Spiny Pocket Mice (Chaetodipus) in Cismontane Southern California. Bulletin of the Southern California Academy of Sciences 98(2): 57-65.
 Huey, L. M. 1964. The Mammals of Baja California, Mexico. San Diego Society of Natural History.
 Brylski, P. ND. San Diego Pocket Mouse. R. Duke, ed. California's Wildlife. Vol. I-III. California Departmet of Fish and Game, Sacramento, California.
 Grinnell, Joseph. 1933. Review of the Recent Mammal Fauna of California, Vol. 40. University of California Press.
 Miller, A. H. and R. C. Stebbins. 1964. The Lives of Desert Animals in Joshua Tree National Monument. University of California Press.
 Patton, J. L.. and S. T. Alvarex-Castaneda. 1999. Family Heteromyidae. In Mamı´feros del noroeste de Mexico (S. T. Alvarez-Castaneda and J. L. Patton, eds.). Centro de Investigaciones Biolo´gicas del Noroeste, La Paz, Baja California Sur, Mexico.
 Lackey, J. A. 1996. Chaetodipus fallax. Mammalian Species 517: 1-6.
 Hafner, J. C. and M. S. Hafner. 1983. Evolutionary Relationships of the Heteromyid Rodents. Great Basin Naturalist Memoirs 7:3-29.
 Hafner, J. C., J. E. Light, D. J. Hafner, M. S. Hafner, E. Reddington, D. S. Rogers, and B. R. Riddle. 2007. Basal Clades and Molecular Systematics of Heteromyid Rodents. Journal of Mammalogy 88(5): 1129-1145.
 Álvarez-Castañeda, S.T., Castro-Arellano, I. , and Lacher, T. 2016. Chaetodipus fallax. The IUCN Red List of Threatened Species 2016. Available: http://www.iucnredlist.org/details/4330/0. Accessed: November 20, 2017.
 Hayden, P., J. J. Gambino, and R. G. Lindberg. 1966. Laboratory Breeding of the Little Pocket Mouse, Perognathus longimembris. Journal of Mammalogy 47:412-423.
 Wilson, D.E. and Ruff, S. 1999. The Smithsonian Book of North American Mammals. Smithsonian Institution Press, Washington, DC, USA.
 Meserve, P. L. 1976. Food Relationships of a Rodent Fauna in a California Coastal Sage Scrub Community. Journal of Mammalogy 57(2): 300-319.
 MacMillen, R. E. 1964. Population Ecology, Water Relations, and Social Behavior of a southern California Semidesert Rodent Fauna.