_Range._--Central Arizona south to Jalisco and including the southern half of Baja California. Marginal occurrences (after Hatfield, _op. cit._: 261, unless otherwise indicated) are: _Arizona_: Camp Verde; Fort Bowie. _Sonora_: Pilares (Burt, Miscl. Publ., Mus. Zool., Univ. Michigan, 39:24, 1938). _Jalisco_: Barranca Ibarra (Miller, orig. descr.). _Baja California_: Miraflores; San Ignacio. _Arizona_: Bates Well. _Diagnosis._--Size small for the species; total length, 67.1(60-72); tibia, 11.3(10.1-12.3); forearm, 28.4(26.3-30.0); greatest length of skull, 11.7(11.3-12.0); breadth of braincase, 6.1(5.9-6.3). Color: between Cinnamon Drab and Drab, dorsally; Wood Brown to Light Drab, ventrally (after Hatfield, _op. cit._:260). _Remarks._--Hatfield (_op. cit._) examined no specimens from Mexico (Baja California excepted) and Burt (_op. cit._) who did examine some specimens (from Sonora), referred one from northwestern Sonora to _P. h. hesperus_ and those from northeastern Sonora to _P. h. merriami_. Since our treatment of subspecies of _Pipistrellus_ (_P. s. veracrucis_ excepted) aims merely to reflect the latest systematic treatment accorded the animals, we would follow Burt (_op. cit._) were it not for the fact that he shows the geographic range of _P. h. merriami_ separated by the range of _P. h. hesperus_ into two parts. This is inconsistent with the ordinarily accepted concept of subspecies. Consequently, we have followed Hatfield (_op. cit._). Clearly, a critical study is needed of adequate material of _Pipistrellus hesperus_ of Mexico. +Pipistrellus hesperus maximus+ Hatfield _Pipistrellus hesperus maximus_ Hatfield, Jour. Mamm., 17:261, August 14, 1936. _Type locality._--Dog Spring, Hidalgo County, New Mexico. _Range._--Southern New Mexico, western Texas and probably the adjoining parts of Mexico. Marginal occurrences (after Hatfield [_op. cit._:261] except as otherwise indicated) are: _New Mexico_: Animas Valley; Florida Mountains; Carlsbad Cave. _Texas_: Mouth of Pecos River (Bailey, N. Amer. Fauna, 25:210, 1905); Boquillas (Borell and Bryant, Univ. California Publ. Zool., 48:9, 1942); Glen Spring (Borell and Bryant, _loc. cit._). _Diagnosis._--Size large for the species; total length, 80.3(78-83); tibia, 12.3(11.7-13.1); forearm, 32.9(31.8-33.3); greatest length of skull, 12.7(12.3-12.9); breadth of braincase, 6.6(6.5-6.7). Color between Smoke Gray and Pale Drab (after Hatfield, _op. cit._:261). +Pipistrellus hesperus santarosae+ Hatfield _Pipistrellus hesperus santarosae_ Hatfield, Jour. Mamm., 17:261, August 14, 1936. _Type locality._--Santa Rosa, Guadalupe County, New Mexico. _Range._--New Mexico (excepting southern part) and western Colorado. Marginal occurrences (after Hatfield, _op. cit._:262) are: _Colorado_: Bedrock. _New Mexico_: Santa Rosa; Socorro; Laguna.