This report is based on the study of approximately 3,520 museum study skins, skulls, complete skeletons, and entire animals preserved in liquid. Most specimens examined were accompanied by an attached label bearing data on locality and date of capture, name of collector, external measurements, and sex. In addition, 49 fossil specimens referable to _Baiomys_ were studied. Nearly two-thirds of the specimens were assembled at the University of Kansas Museum of Natural History; the remainder were examined in other institutions. Specimens studied were grouped by geographic origin, sex, age, and season of capture. Individual variation was then measured in several of the larger samples of each living species and in measurable fossil material. External measurements used were those recorded by the collectors on the labels attached to the skins. Twenty cranial measurements employed in the past in the study of _Baiomys_ and closely related cricetine rodents were statistically analyzed. The coefficient of variation was calculated for each of the 20 measurements in order to determine which varied least. In general, measurements having the least coefficient of variation were used in comparing samples from different geographic areas. Figure 1 shows the points between which measurements were taken. _Occipitonasal length._--From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone. _A_ to _A'_ _Zygomatic breadth._--Greatest distance across zygomatic arches of cranium at right angles to long axis of skull. _B_ to _B'_ _Postpalatal length._--From posterior margin of hard palate to anterior margin of foramen magnum. _C_ to _C'_ _Least interorbital breadth._--Least distance across top of skull between orbits. _D_ to _D'_ _Length of incisive foramina._--From anteriormost point to posteriormost point of incisive foramina. _E_ to _E'_ _Length of rostrum._--The distance in a straight line from the notch that lies lateral to the lacrimal to the tip of the nasal on the same side. _F_ to _F'_ _Breadth of braincase._--Greatest distance across braincase, taken at right angles to long axis of skull. _G_ to _G'_ _Depth of cranium._--The distance from the dorsalmost part of the braincase to a flat plane touching tips of incisors and ventral border of each auditory bulla. A glass slide one millimeter thick was placed on the ventral side of the skull. One jaw of the caliper was on the lower surface of the slide and the other jaw on the dorsalmost part of the braincase. The depth of the slide was subtracted from the total reading. _H_ to _H'_ _Alveolar length of maxillary tooth-row._--From anterior border of alveolus of M1 to posterior alveolus of M3. _I_ to _I'_ [Illustration: FIG. 1. Three views of the skull to show points between which measurements were taken. Based on _B. m. pullus_, adult, female, No. 71611 KU, 8 mi. S Condega, Esteli, Nicaragua. x 1-1/3.]