_Geographic variation._--In the Old World four subspecies are currently recognized (see Allen, 1933:316) and the same number is here recognized in North America. Length of the tail, length of head and body and hind foot, breadth of the rostral part of the skull in relation to its length, and position on the side of the head of the line of demarcation between the dark color of the upper parts and the white underparts, are the features in which geographic variation has been detected. The general impression is that the amount of geographic variation is much less than in _Mustela frenata_ and only slightly less than in _Mustela erminea_ of the same geographic area. _Nomenclature._--It is exceptional for a species which occurs in both the Old-and New-World to take its specific name from New World material, especially if the name was proposed as recently as 1896; most circumboreal species take their names from descriptions of European specimens. Although the least weasel, _Mustela rixosa_ (Bangs) 1896, seems now to be an exception, it may yet turn out that the first available name was based on European material. Zimmermann (1943) shows that the least weasel actually was named on the basis of European material long before 1896 and concludes that the name _Putorius minutus_ Pomel, 1853, based on a specimen from France, is the first available name. Because _Putorius_ nowadays is relegated to subgeneric rank under the generic name _Mustela_, we have for consideration the name-combination _Mustela minuta_ (Pomel). Unfortunately for Zimmermann's conclusion, _Mustela minuta_ Pomel is not available because it is preoccupied by _Mustela minuta_ Gervais [= _Palaeogale minuta_ (Gervais), 1848-1852--see Simpson, 1946: 2, 12], a name applied to another species of small mustelid from the Oligocene or lower Miocene deposits of Europe. Some other early names thought by Zimmermann (1943:290) to have been based on the dwarf weasel of Europe are judged to be _nomina nuda_ and therefore are to be ignored.