In the present account it is aimed to record what is known concerning geographic distribution, taxonomically significant characters, and interrelationships of the known kinds as well as to provide means for recognizing the species and subspecies in the genus. In addition, attention is given to the probable center of origin of the subfamily Zapodinae and to the relationships and taxonomic positions of the genera _Zapus_, _Napaeozapus_, and _Eozapus_. MATERIALS, METHODS, AND ACKNOWLEDGMENTS The present report is based on a study of approximately 3,600 specimens that were assembled at the Museum of Natural History of the University of Kansas or that were examined at other institutions. Most of these specimens are stuffed skins with skulls separate. Skulls without skins, skins without skulls, entire skeletons, and separately preserved bacula are included as a part of the total. Almost every specimen is accompanied by an attached label, which bears place and date of capture, name of collector, external measurements, and sex. Specimens used in the study of geographic variation were arranged by season of capture and according to geographic location; then they were segregated as to sex, and, under each sex, by age. Next, individual variation was measured in comparable samples of like age, sex, season, and geographic origin. Finally, comparable materials were arranged geographically in order to determine variations of systematic significance. The only external measurements used were total length, length of tail, and length of hind foot; these measurements were recorded by the collectors on the labels attached to the skins. Height of the ear was not used since it was not recorded by many of the collectors. In order to determine which cranial structures showed the least individual variation but at the same time showed substantial geographic variation, a statistical analysis was made of the 30 measurements, of cranial structures, heretofore used in taxonomic work on _Zapus_. The following measurements of the skull showed the least individual variation but showed some geographic variation and therefore, were used in this study. See figs. 1-3 which show points between which measurements were taken: _Occipitonasal length._--From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone. _a_ to _a´_ _Condylobasal length._--Least distance from a line connecting posteriormost parts of exoccipital condyles to a line connecting anteriormost projections of premaxillary bones. _b_ to _n_ _Palatal length._--From anterior border of upper incisors to anteriormost point of postpalatal notch. _b_ to _b´_ _Incisive foramina, length._--From anteriormost point to posteriormost point of incisive foramina. _c_ to _c´_ _Incisive foramina, breadth._--Greatest distance across incisive foramina perpendicular to long axis of skull. _f_ to _f´_