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_.
Intergradation with _Zapus princeps oregonus_ was noted by Davis (1939:340) in a specimen from Cedar Mountain in Idaho. I have not seen this individual which he referred to _Z. p. idahoensis_ but have seen a specimen from the N Fork of Potlatch River (15 mi. SE Cedar Mt.), which, in color, closely resembles _Z. p. oregonus_ but cranially (shape of incisive foramina, size, and inflation of auditory bullae) is more nearly like _Z. p. idahoensis_ to which it is referred. Davis (_loc. cit._) indicates that specimens from summit of Smith Mt., from 1 mi. N Bear Creek R. S., from 1/2 mi. E Black Lake, and from 3 mi. W Payette Lake, Idaho, are in an area of intergradation between _Z. p. oregonus_ and _Z. p. idahoensis_, but he referred them to _Z. p. idahoensis_ on the basis of cranial characters and length of hind foot. Seven specimens from Alturas Lake, 7000 ft., Idaho, were likewise so allocated by Davis (_loc. cit._). I concur with him and in addition refer the following intermediate individuals from Idaho to _Z. p. idahoensis_: New Meadow, 1; Warren, 1; Perkins Lake, 7000 ft., Sawtooth Nat'l Forest, 1; Prairie Creek, 12 mi. W Ketchum, 2400 ft., 3. All are more nearly like _Z. p. oregonus_ in color but cranially they show more resemblance to _Z. p. idahoensis_.
In the eastern part of the range of _Z. p. idahoensis_, intergradation occurs with _Zapus princeps minor_, as at 15 mi. S Heath, N Fork Flat Willow Creek, Big Snowy Mt's, Montana. Specimens from there have the lateral line enlarged and the maximum seen in this species of Ochraceous color ventrally. The pterygoid fossae are large and the bullae are reduced as in _Z. p. minor_, but in the sum total of the characters the mice more closely resemble _Z. p. idahoensis_. At Lewistown, 7 mi. NE Judith Mt's, Lime Kiln Gulch, Montana, the animals are colored as are _Z. p. minor_ but cranially are like _Z. p. idahoensis_ to which they are referred. Specimens from the Highwood Mt's, Montana, also are intergrades; they have a relatively distinct lateral line as in _Z. p. minor_ but show no ventral suffusion of Ochraceous; they have large bullae, nasals that are straight in lateral profile and other cranial characters of _Z. p. idahoensis_ to which they are here referred.