The Pennsylvania meadow mouse is an abundant mammal along the Alaska Highway. Alcorn obtained specimens at most of his trapping stations, frequently in company with _Microtus oeconomus_ at the more northern localities. A preferred habitat was grassy areas and willow clumps along streams or at the edges of lakes. The best catches were made along well-used runways, especially where there were piles of cut grass. These runways were used also by _Clethrionomys_ and other small animals. Specimens of _M. pennsylvanicus_ were frequently taken in the daytime; one was taken on June 29 as it was swimming at the edge of a small lake near the junction of the Liard River and Irons Creek in British Columbia. Lacking sufficient comparative material in the past, most workers have considered that _M. pennsylvanicus_ ranges without appreciable geographic variation throughout most of northwestern Canada and Alaska, where it has been referred to the subspecies, _M. p. drummondii_. Dale (1940), in studying collections made in British Columbia and southeastern Alaska, found evidence of geographic variation and recognized two new subspecies; thus he not only pointed out geographically variable characters but reduced the size of the range ascribed to _M. p. drummondii_. A later work by Rand (1943) considered the northwestern populations of _M. pennsylvanicus_ as being too variable to show distinctive groupings. The large collection made by Alcorn offers evidence that other separable subspecies with constant characters are present. Study of this material indicates the presence of two unnamed subspecies, which are named and described as follows: Microtus pennsylvanicus alcorni new subspecies _Type._--Female, adult, skin with skull, No. 21552, Univ. Kansas, Mus. Nat. Hist., 6 mi. SW Kluane, 2550 feet elevation, Yukon Territory, Canada; 24 August 1947; obtained by J. R. Alcorn; original No. 5240. _Range._--Extreme southwestern Yukon Territory and adjacent parts of Alaska as far south as Haines, as far north as Northway, and as far west along the Alaskan coast as Anchorage and Tyonek. _Diagnosis._--Size large (see measurements); color of upper parts near (_l_) Brussels Brown; skull noticeably ridged; zygomatic arches heavy, rounded and relatively short; rostrum heavy; auditory bullae not greatly expanded; maxillary teeth relatively heavy and low-crowned. _Comparisons._--From _M. p. drummondii_ (specimens from vicinity of Whitehorse, Y. T., Trutch, B. C., and Kinuso, Alberta), _M. p. alcorni_ differs as follows: Averaging larger in all measurements taken except lengths of tail and hind foot, which are the same; color of upper parts slightly paler and more gray and less brown; underparts paler; zygomatic arches heavier, rounder and shorter; skull proportionately more massive, except the auditory bullae which are less inflated; maxillary teeth heavier and lower-crowned.