The present paper is a first step in this larger investigation.[2] In this work the authors have made no attempt to deal with the taxonomic side of the kangaroo rat problem. It is not unlikely that intensive studies will show that the form now known as _Dipodomys spectabilis spectabilis_ is made up of a number of local variants, some of them perhaps worthy of recognition as additional subspecies. But it is felt that the conclusions here reached will be little, if at all, affected by such developments. Color descriptions are based on Ridgway's Color Standards and Color Nomenclature published in 1912. [Footnote 1: References in parentheses are to the Bibliography, p. 40 (the last figure being to the page of the publication). References to authorities where no citation of literature is appended relate for the most part to manuscript notes in the files of the Biological Survey or the University of Arizona Agricultural Experiment Station.] [Footnote 2: In addition to assistance rendered by officials of the Biological Survey and the University of Arizona, which is hereby acknowledged, the authors are indebted to the following persons for helpful suggestions and assistance: G. S. Miller and J. W. Gidley, of the U. S. National Museum; Dr. Frederic E. Clements and Gorm Loftfield, of the Carnegie Institution; Morgan Hebard, of the Academy of Natural Sciences of Philadelphia; James T. Jardine and R. L. Hensel, both formerly connected with the U. S. Forest Service; and R. R. Hill, of the Forest Service. They are also indebted to William Nicholson, of Continental, Ariz., for many courtesies extended in connection with work on the Reserve.] IDENTIFICATION. There are only three groups of mammals in the Southwest having external cheek pouches. These are (_a_) the pocket gophers (Geomyidae), which have strong fore feet, relatively weak hind feet, and short tail, as compared with weak fore feet, relatively strong hind feet, and long tail in the other two; (_b_) the pocket mice (_Perognathus_), which are considerably smaller than the kangaroo rats and lack the conspicuous white hip stripe possessed by all the latter; and (_c_) the kangaroo rats (_Dipodomys_). [Illustration: FIG. 1.--Range, east of the Colorado River, of _Dipodomys spectabilis spectabilis_ compared with that of _Dipodomys merriami_. Cross hatching indicates area of overlapping of the two forms. The range of _Dipodomys deserti_, not shown on the map, is west of that of _spectabilis_, and so far as known the two do not overlap.]