The descent of the Vashon-Wisconsin continental glacier eliminated all pocket gophers north of the drift border in the state of Washington. At the maxinium extent of the ice sheet, gophers were present in the southern Cascade Mountains, on the Columbian Plateau, and in southeastern Washington. The dark-colored gophers of southeastern Washington were connected to those of the southern Cascade Mountains by a humid "bridge" along the Simcoe Anticline. This bridge, which was occupied by dark-colored gophers, isolated the pale gophers of the Columbian Plateau. With the retreat of the glaciers the Simcoe Bridge became more arid and no longer served as a geographically continuous habitat to connect the two dark-colored populations. The pale gophers of the Columbian Plateau then extended their range south, along the valley of the Columbia River. Gophers from Idaho invaded the land north of the drift border, recently cleared of ice, and extended their range to the west and south until they nearly encircled the pale gophers on the Columbian Plateau. The dark gophers of the southern Cascades pushed their range westward, on glacial outwash trains, to the glacial prairies of the Puget Sound area, the Olympic Mountains, and to river-terrace prairies in southwestern Washington. The fossorial habits of pocket gophers, their limited tolerance with regard to ecologic conditions, and their ability to exist for long periods of time in a single locality, all combine to make gophers subject to isolation by barriers that ordinarily do not restrict the movements of other vertebrates. As one result, pocket gophers within areas circumscribed by the more rigid barriers exist as independent populations in areas suitable to their needs. Such isolated populations behave as small, independent units and, as a result of different selective factors and the chance fluctuations of genetic factors, develop into microgeographic races. As a whole, the barriers separating these microgeographic races are constantly breaking down and reforming. Thus, isolation and intermingling of populations are constantly going on. Characters developed in microgeographic races might eventually spread throughout a subspecies as a result of successive chance "conquests" of other small populations or as a result of natural selection. Color was studied in the seventeen races of gophers from the entire state of Washington. Differences in the color of the various races were found to be due principally to variations in the amount or kind of pigment present throughout the entire length of each hair. The variations in the amount or kind of this pigment are thought to be due to chance combinations of alleles of a multiple factor series. The gross color is modified by the relative length of the sepia-colored proximal band of the fur. As a result of our now clearer understanding of the immigrational history and microgeographic variation of the pocket gophers of Washington, and in the light of our tentative genetic analysis of the variations in size, color and skull, we conclude that seventeen subspecies, all belonging to the species Thomomys talpoides, occur in the state. Eight isolated populations of pocket gophers in the Puget Sound area, belonging to five subspecies, came from a common stock in post-Pleistocene times and now occupy similar but isolated habitats. Large samples of these populations were collected and studied in an effort to correlate differences in their habitats. Quantitative differences in size and cranial construction were found to be correlated with the depth of the soil in the habitat occupied. Depth of soil seems to act as a selective factor in the fixation or loss of genetic factors, already present or newly acquired, in a multiple factor series for size. No correlation between the color of the gophers of the Puget Sound area and any measurable feature of their environments was found. It is concluded that selective factors for color are less strong than those for size and quantitative cranial characters.