While the Wisconsin ice was at its maximum extent, _Microtus montanus_ from the Blue Mountains crossed the Simcoe Bridge to the Yakima Valley and the eastern Cascade Mountains. The closing of the Simcoe Bridge isolated these mice, which subsequently became slightly differentiated, as _canescens_. Another stock moved westward along the Columbia River to western Oregon. This stock is now called _Microtus canicaudus_ but is probably racially rather than specifically distinct from _Microtus montanus nanus_. With the retreat of the Wisconsin ice, _montanus_ extended its range northward from the Yakima Valley along the eastern Cascade Mountains to extreme southern British Columbia. _Microtus pennsylvanicus funebris_ entered northeastern Washington with other elements of the Rocky Mountain Fauna. At present, _pennsylvanicus_ occurs in northeastern Washington while _montanus_ is found in southeastern Washington and the eastern Cascade Mountains. The Okanogan River Valley separates their ranges. Eventually _montanus_ may extend its range to northeastern Washington and _pennsylvanicus_ to the Cascades, the two forms occurring together as they do in Montana, Idaho and Colorado. The isolation of _kincaidi_ on the Columbian Plateau seems complete and the probability of its range reaching that of its related species seems slight. MICROTUS LONGICAUDUS.--The later distributional history of the long-tailed meadow mouse is not yet clear. MICROTUS TOWNSENDII.--This Pacific Coastal species probably lived west of the Sierra Nevada-Cascade Mountains since the early Pleistocene. It has extended its range northward since the retreat of the Vashon ice and has reached some of the islands in Puget Sound and the Strait of Georgia. This species seems to be related to _Microtus richardsoni_ but the two species probably separated at an early time. MICROTUS RICHARDSONI.--The water rat of the Cascade Mountains seems to be have been forced southward into Washington and Oregon by the descent of the Vashon-Wisconsin glaciers where it became isolated from the water rat of the northern Rocky Mountains. After the retreat of the ice, both forms have moved northward. Bailey (1900) records _macropus_ from Bonner County, Idaho, not far from northeastern Washington but in Washington up to this time, _macropus_ has been recorded only from the Blue Mountains of the southeastern part of the state. MICROTUS OREGONI.--The later historical distribution of the creeping mouse was probably the same as that of _townsendii_. LAGURUS CURTATUS.--The distribution of this species of the Great Basin Fauna has probably changed little if any since the late Pleistocene.