Female (based on the type specimen): See measurements and plates 31-33; weight, 1.6 grams; basilar length, 35.5; zygomatic breadth less than distance between condylar foramen and M1 and more than distance between anterior palatine foramen and anterior margin of tympanic bulla (nearly equal in each instance); postorbital breadth less than length of upper premolars and greater (7.1-8.4) than width of basioccipital measured from medial margin of one foramen lacerum posterior to its opposite; least width of palate equal to inside length of P4; tympanic bulla as far posterior to foramen ovale as width of 3 (including I3) upper incisors; height of tympanic bulla more than distance from its anterior margin to foramen ovale; length of tympanic bulla more than length of lower molar-premolar tooth-row and greater than length of rostrum. The skull of the female averages 41 per cent lighter than that of the male. Compared with the skull of _M. f. nevadensis_, that of _arizonensis_ is smaller, less heavily ridged and has more inflated tympanic bullae and a relatively greater mastoid breadth. Comparison with the skull of _M. f. neomexicana_ is made in the account of that subspecies. _Remarks._--In 1891 Mearns (234-235) named this weasel as a full species on the basis of two individuals taken by him in 1886 and 1887. Since that time only a few additional specimens have been preserved. Only four are adults. Although this material does not permit of a definition of the subspecies as precise as could be wished, still, it clearly shows that the animals from the plateau region of Arizona are recognizably different from those farther north in the Sierra Nevada of California and those of the Rocky Mountains and Great Basin region northward to the Canadian border. These more northern animals have gone by Mearns' name, _arizonensis_, since the date of its proposal until 1939 when the name _nevadensis_ was proposed. The smaller size, especially of the skull, and the greater inflation of the tympanic bullae are the outstanding characters which distinguish _arizonensis_ from the similarly marked _nevadensis_. The bullae are relatively much inflated throughout but especially so on the posteromedial parts. Although the three adult males and two subadult females available of this subspecies are smaller in most parts measured than any of the scores of _nevadensis_ of similar age that have been measured, overlap in size probably will be found as additional specimens of _arizonensis_ become available. A young female, no. 18513, coll. D. R. Dickey, from Little Spring, does have certain cranial measurements as large as are found in the minimum-sized _nevadensis_ from farther north.