_Type._--Obtained at Astoria, Clatsop County, Oregon, by J. K. Townsend in 1836; type in Philadelphia Academy of Natural Sciences. _Measurements._--Ten males and 10 females from southwestern Washington, average, respectively: total length 133, 126; length of tail 35, 32; hind foot 16.9, 16.7; ear 9.8, 9.7; weight 19.3, 19.1 grams. _Distribution._--The Cascade Mountains, the Olympic Mountains, and the lowlands of western Washington. _Remarks._--A large series of topotypes of _Microtus o. oregoni_ in the California Museum of Vertebrate Zoölogy, and the rather large series from Cowlitz County, Washington, show a wide range of variation in color, size, and cranial characters. Specimens from the Cascade and Olympic Mountains seem to average a bit paler than topotypes of _oregoni_, but are not worthy of recognition as distinct races. Too few topotypes of _Microtus o. serpens_ from British Columbia are available to judge the status of that race with any certainty, but specimens from northwestern Washington are certainly _oregoni_. _Description._--The creeping mouse is a small species of general microtine form. The head and body measure about 4 inches and the tail about 1-1/2 inches. The ears are small and the eyes tiny, nearly buried in the fur. The fur is short and rather rough. It does not lie back smoothly, thereby giving a woolly appearance. The upper parts are reddish or grayish brown and the underparts are grayish white. The nondescript appearance suggests a young rather than adult mouse. The creeping mouse (subgenus _Chilotus_) is restricted to the Pacific Coast and ranges from British Columbia to California. In Washington it occupies almost every conceivable "mouse" habitat in its range, including wet marshes, damp ravines, dry forest, damp, mossy forest, meadows, alpine meadows and fields of short grass. It is rare in all but the latter habitat. In fields of short or dry grass it is often abundant. In the Cascade Mountains it was in relatively dry places along streams or rock slides. Altitudinally it ranges from sea level to at least 6,000 feet, and from the Humid Transition well into the Hudsonian life-zones. Creeping mice construct tiny tunnels among the grass roots and seldom venture out of them. In suitable habitat the surface of the ground beneath the grass is a maze of these tunnels, which cross, intersect, and divide in a complex network. An observer standing in a field occupied by creeping mice can scarcely conceive of the extent and perfection of the tiny tunnel system at his feet. Creeping mice lived but a day or two in captivity. Save for the bits of grass blades left in their runways, little is known of their food or other life habits. Their nests are round balls of dry grasses placed in cavities under logs. None of the many examined possessed a lining of softer materials. Embryos found in pregnant females from April 10 to May 18 numbered from 2 to 4.