The pelage of _Myotis velifer brevis_ is shorter than that of either _M. v. velifer_ or _M. v. incautus_ and gives the impression of being less dense. The dorsal hairs average approximately 4.5 millimeters long in _M. v. brevis_ taken 35 miles north of Blythe, Riverside County, California, in May, eight millimeters in _M. v. velifer_ collected at Las Vigas, Veracruz, in January, and six millimeters in _M. v. incautus_ taken four and one half miles southwest of Sun City, Barber County, Kansas, in November. More than seasonal differences in length of pelage is indicated by measurements of additional specimens of each subspecies taken at different times of the year. Considering its extensive geographic range and its occurrence in many contrasting environments, _Myotis velifer_ varies little; and the variation that does occur is continuous. The change from the large, dark Mexican subspecies to the small, pale Arizonan subspecies is gradual. The reason may lie in the ecology of _M. velifer_. It seems that there are few barriers separating populations. Waterless areas and regions lacking suitable roosting places such as fissures in cliffs and outcrops of rocks, caves and buildings, may exclude the species from certain areas, but there are few areas of any great size within the range of the species that lack these features. Also, these bats are strong fliers; even between fairly distant colonies there may be considerable gene flow. The geographic variation observed probably is the result of adaptation, on the part of populations in different parts of the range of the species, to different environments. The lack of any effective barriers except possibly distance between populations tends to limit subspeciation and to cause gradual variation between subspecies. _M. v. peninsularis_, the subspecies at the southern tip of Baja California, is not considered in this discussion. So far as known, however, that subspecies is completely isolated from the mainland populations of _M. velifer_. For the opportunity to examine specimens under their care I wish to thank Dr. William H. Burt of the University of Michigan Museum of Zoology, Dr. Rollin H. Baker of the Museum of Natural History of the University of Kansas, and Dr. Donald F. Hoffmeister of the University of Illinois Museum of Natural History. I am indebted also to persons in charge of the Biological Surveys Collection and the National Museum for the loan of critical material, and to Dr. E. Raymond Hall for suggestions. The following symbols are used to designate the source of specimens: BS--Biological Surveys Collection, IM--University of Illinois Museum of Natural History, KU--Museum of Natural History of the University of Kansas, MM--University of Michigan Museum of Zoology, NM--United States National Museum, TV--collection of Terry A. Vaughan. _Specimens examined._--Total, 110, distributed as follows: