Female: One young from the summit of the Trinity Mountains east of Hoopa and one nontypical adult from 5500 feet elevation on South Fork Mountain, Humboldt County, measure respectively as follows: Total length, 330, 325; length of tail, 115, 123; length of hind foot, 37, 37. Tail is 53 and 61 per cent as long as head and body. Length of hind foot less than basal length. Average differences in external measurements between the two sexes, indicated by the unsatisfactory material available, are: Total length, 86; length of tail, 31; length of hind foot, 9. _Externals._--Longest facial vibrissae black or dark brown and extending beyond ear; carpal vibrissae same color as underparts and extending as far as apical pad of fifth digit; hairiness of foot-soles, in summer pelage, as shown in figure 19. _Color._--Upper parts, in summer, Brussels Brown to near (_n_) Brussels Brown or lighter than tone 3 of Raw Umber of Oberthür and Dauthenay, pl. 301, usually darkest on nose and forehead. Chin white. Remainder of underparts Buff-Yellow to Warm Buff. Tip of tail black. Winter pelage unknown. Color of underparts extends distally on posterior sides of forelegs over toes onto antipalmar faces of feet and sometimes wrists, on medial sides of hind legs only to ankles, but toes sometimes with isolated white markings. Least width of color of underparts in the type and 2 adults from Jackson Lake averaging 35 (30-40) per cent of greatest width of color of upper parts. Black tip of tail averaging 54 (53-55) mm. long; thus longer than hind foot and averaging 37 per cent of length of tail-vertebrae. _Skull and teeth._--Male (based on 4 adults: Type, Mt. Ashland, 1; Jackson Lake, 2): See measurements and plates 19-21, 30. As described in _Mustela frenata nevadensis_ except that: Weight, 3.8 (3.5-4.3) grams; basilar length, 44.4 (42.6-45.8); zygomatic breadth more or less than distance between condylar foramen and M1 or than distance between anterior palatine foramen and anterior margin of tympanic bulla; mastoid breadth more than postpalatal length; least width of palate less than medial length of P4 (except in one specimen). Female (based on one adult possibly not typical, from 5500 ft., South Fork Mt.): See measurements. As described in _Mustela frenata nevadensis_ except that: Weight, 2.2 grams; basilar length, 38.1; zygomatic breadth less than distance between condylar foramen and M1 and less than distance between anterior palatine foramen and anterior margin of tympanic bulla; postorbital breadth more than width of basioccipital measured from medial margin of one foramen lacerum posterior to its opposite.