From _Zapus princeps minor_, _Z. p. idahoensis_ differs in: Size larger; color of underparts less ochraceous; lateral line indistinct or wanting; skull averaging larger in all measurements taken except that the two subspecies are approximately same in least interorbital constriction, length of zygomatic arch, and distance from anterior face of incisors to postpalatal notch; nasals, in profile, straight instead of with proximal third depressed; postpalatal notch anterior to posterior face of last molar, instead of even with, or usually posterior to, same. From _Zapus princeps saltator_, _Z. p. idahoensis_ differs as follows: Size averaging slightly larger; color darker, being less ochraceous and more yellow dorsally and laterally; auditory bullae more inflated; zygomatic arches less bowed laterally; incisive foramina narrower. For comparison with _Zapus princeps princeps_ and _Zapus princeps cinereus_ see accounts of those subspecies. _Remarks._--Intergradation occurs at almost all of the places where the range of _Z. p. idahoensis_ is known to touch that of any other geographic race. Nevertheless, each of the populations studied has characters which make this subspecies recognizable as a taxonomic unit, although its characters are not yet stabilized even in the central part of its range. Among named subspecies of _Zapus princeps_, _Zapus p. idahoensis_ most closely resembles _Zapus princeps kootenayensis_, its nearest geographic neighbor to the north. Three specimens from 2 mi. NE Weippe, 3000 ft., Idaho, are best referred to _Z. p. idahoensis_ but show relationship to _Z. p. kootenayensis_ in size and shape of the tympanic bullae. The relationship of individuals from Idaho, here referred to _Z. p. idahoensis_, from Glidden Lakes, Enaville, Cascade Creek, and 13 mi. E and 5 mi. N Coeur d'Alene, is discussed in the account of _Z. p. kootenayensis_. British Columbian specimens from Newgate and Crows Nest Pass, 4450 ft., as well as Albertan specimens from Crows Nest Pass and various places in Waterton Lake Park, resemble _Z. p. kootenayensis_ in color but cranially are more nearly like _Z. p. idahoensis_.