From _Zapus princeps idahoensis_, _Z. p. princeps_ differs in: Size larger; darker with more Ochraceous-Buff; lateral line much more distinct; underparts frequently suffused with Ochraceous-Buff rather than seldom so; skull larger as regards length of palatal bridge, length of zygomatic arch, and width of proximal part of inferior ramus of zygomatic process of maxillary; pterygoid fossae broader; medial projection on inferior ramus of zygomatic process of maxillary large instead of reduced or absent; postpalatal notch usually anterior to, or on a plane with, posterior face of last molars rather than posterior to same. _Remarks._--This subspecies retains most of its diagnostic characters in all parts of its geographic range. An individual from the type locality, Florida, Colorado, resembles _Zapus princeps luteus_ in color, but cranially is most nearly like _Z. p. princeps_. A specimen from Tierra Amarilla, New Mexico, a locality 25 miles north of, and in homogeneous habitat with, El Rito, New Mexico, from which specimens of _Z. p. luteus_ are known, shows resemblance to the latter in some cranial characters (see account of _Zapus princeps luteus_) but is most nearly like _Z. p. princeps_ to which it is referred. Animals from Medicine Wheel Ranch, 9000 ft., 28 mi. E Lovell, Wyoming, which are here referred to _Z. p. princeps_, show intergradation with _Zapus princeps idahoensis_, being similar in size of pterygoid fossae, breadth of postpalatal notch, and in size and degree of inflation of the auditory bullae, but differ in color and in other cranial characters. Specimens from 2 mi. E Shriver, 6500 ft., Montana, which lack the distinct lateral line and ventral suffusion of Ochraceous-Buff, are here referred to _Z. p. idahoensis_. _Specimens examined._--Total, 344, distributed as follows: