Intergradation between _gambelii_ and _artemisiae_ is normal and takes place gradually where the ranges of the two subspecies meet. The same is true of intergradation between _oreas_ and _gambelii_ in the eastern Cascade Mountains. West of the Cascade Mountains the range of _oreas_ meets the range of _austerus_. These two subspecies appear not to interbreed in nature, since no intergrades were taken at any of the numerous localities in western Washington where the mice were trapped. Pure populations of the two subspecies exist within a few miles of each other. In the valley of the Skykomish River, in the western Cascades Mountains, from the town of Skykomish, King County, to the lowlands to the west, only _austerus_ was found. In the coniferous forests of the mountains above Skykomish, only _oreas_ was taken. Several pairs, each an _oreas_ and an _austerus_ of the other sex, were kept from four to six months, and one pair was kept for a year, but they did not reproduce. The _oreas_ were from the upper Skykomish Valley and the _austerus_ were from Seattle, King County. Along the border of the ranges of the two subspecies in the Skykomish River Valley, a definite habitat preference was noted. The coniferous forests were occupied by _oreas_ and brush or deciduous forests by _austerus_. Within the range of _austerus_ and within the range of _oreas_ only one subspecies is found whether the habitat be coniferous forest or thickets of alder and willow, but where the ranges of the two subspecies meet _austerus_ occurs only in the thickets of aspen and willow and _oreas_ occurs only in the coniferous forest. The morphological characters that separate _oreas_ from _austerus_, namely, larger size, richer color, and longer tail, are all features that might be associated with a more arboreal existence in dark, coniferous forests. Our observations show that _oreas_ is, to a large extent, arboreal. Traps nailed to tree trunks six to eight feet from the ground and set for flying squirrels often caught _oreas_ in the Skykomish River Valley. On one occasion I walked up a leaning tree trunk to set a trap, fifty feet from the ground on the trunk of another tree that was upright. An _oreas_ was found in this trap the next morning. Svihla (1933: 13) relates how a specimen of _oreas_ that escaped from a live trap took refuge in a tree when pursued. We have set numerous traps for flying squirrels in the area about Puget Sound. As far as memory and field notes serve, we have never taken _austerus_ in these traps. Our observations on the habits of _austerus_ are much more abundant than those on _oreas_, but for _austerus_ no evidence of arboreal activities has been obtained.