In the United States National Museum in July, 1949, the specimen providing the easternmost record station of occurrence was examined by us. This is No. 23591, in alcohol, taken on August 24, 1890, by William Lloyd, original No. 88, at the mouth of the Pecos River in Texas. In the same collection there is a specimen of _Pipistrellus subflavus_ providing the westernmost record of occurrence of that species. This specimen, a skin with skull, is No. 126729, [Male], taken on May 3, 1903, by Jas. H. Gaut, original No. 1271, at Comstock, Texas. The two localities concerned are in the Valley of the Rio Grande, and are only about five miles apart. Nevertheless, the two specimens are clearly referable to their respective species and show no tendency toward intergradation. Consequently, confidence is felt in treating _Pipistrellus hesperus_ and _Pipistrellus subflavus_ as two distinct species. The most recent report upon geographic variation throughout the entire species, _Pipistrellus hesperus_, was that by Hatfield (Jour. Mamm., 17:257-262, August 14, 1936). Later, as explained below in the account of _P. h. australis_, Burt (Miscl. Publ., Mus. Zool., Univ. Michigan, 39:25, February 15, 1938) examined specimens from Sonora, Mexico, and for them and for specimens from southern Arizona proposed a different nomenclatural arrangement. [Illustration: FIG. 1. Map showing the geographic ranges of species and subspecies of _Pipistrellus_. 1. _Pipistrellus h. hesperus_ 2. _Pipistrellus h. merriami_ 3. _Pipistrellus h. australis_ 4. _Pipistrellus h. maximus_ 5. _Pipistrellus h. santarosae_ 6. _Pipistrellus s. subflavus_ 7. _Pipistrellus s. obscurus_ 8. _Pipistrellus s. veracrucis_ ] +Pipistrellus hesperus hesperus+ (H. Allen) _Scotophilus hesperus_ H. Allen, Smithsonian, Miscl. Coll., No. 165, Vol. 7 (art. 1): p. 43, June, 1864. _Vesperugo hesperus_ True, Proc. U. S. Nat. Mus., 7:602, 1885. _Pipistrellus hesperus_ Miller, N. Amer. Fauna, 13:88, October 16, 1897. _Type locality._--Old Fort Yuma, Imperial County, California, on right bank of Colorado River, opposite present town of Yuma, Arizona.