Compared with _M. f. nevadensis_, the skull of the male of _washingtoni_ averages more slender, as shown by the mastoid and zygomatic breadths and has the preorbital part longer, on the average, as shown by the greater ratio (to the basilar length) of the length of the tooth-rows and orbitonasal length. Also, on the average, the postorbital constriction is longer than in _nevadensis_ and the tympanic bullae are smaller. In females, the skull is lighter, the tooth-rows are shorter, the tympanic bullae are smaller, and the preorbital part of the skull is shorter and narrower as shown by the orbitonasal length and interorbital breadth. Except that the tympanic bullae are actually, although not relatively, smaller in males of _effera_, it differs from _washingtoni_ in the same way as does _nevadensis_ as regards relative proportions, but, of course, the actual difference in size is greater since _effera_ is smaller than _nevadensis_. Comparison of the skull with that of _oregonensis_ is made in the account of that subspecies. _Remarks._--_M. f. washingtoni_ was described and named in 1896 by Merriam as a distinct species. Subsequently, specimens which here are regarded as intergrades between _altifrontalis_ and _nevadensis_, were classified as _washingtoni_. The external measurements given for the specimens from Mount Adams are those recorded on the labels in inches and fractions thereof. Instead of total length there sometimes is written "tip to tip." In the series of 19 winter-taken topotypes the hairs project beyond the end of the caudal vertebrae for an average distance of 28 (19-40) millimeters. If the hairs on the end of the tail were included in the measurements, 28 millimeters should be subtracted from the averages. Probably the measurements should stand as given, since an adult male topotype, no. 226758, U. S. Nat. Mus., taken subsequently by Walter P. Taylor measures 405; 152; 51. _Mustela frenata washingtoni_ is not a strongly marked geographic race. In many features it is intermediate between _M. f. altifrontalis_ and _M. f. nevadensis_. This is especially true of coloration. In the series from Mount Adams and that from Mount Rainier, some individuals have the light color of the underparts extended down the hind legs over the feet and over the proximal face of the ventral third of the tail as in _nevadensis_, whereas others from the same place have the light color of the underparts absent from the tail and extending no farther down the hind limbs than the knees. The light color of the underparts in the series of topotypes is so restricted that the transverse extent at the narrowest place amounts to only 24 (10-37) per cent of the greatest width of the color of the upper parts. This narrowness of the color of the underparts has been likened by Merriam (1896:18) to the condition in _Mustela frenata noveboracensis_. So it is, but it is similar to the condition found also in the geographically adjoining _M. f. altifrontalis_.