This account is based upon four years observations of the small mammal population in a small, dark, damp ravine, west of the Cascade mountains in the state of Washington. Such ravines are a typical feature of the humid transition zone of the state and are found wherever a small stream of water flows through dense coniferous forest. The ravines contain a flora and fauna that is both quantitatively and qualitatively different from that of the coniferous forests. The ravine studied is about five acres and situated on the University of Washington Campus at Seattle. It is bordered on three sides by dry hillsides covered with western red cedar, western hemlock, and douglas fir. The eastern, or downstream, end of the ravine was at one time open to Lake Washington and its large bordering marshes, but a railroad embankment, railroad tracks, and a paved highway now close this end. At present the ravine is comparatively isolated. The flora of the ravine consists of broad leaf maple, red cedar, and alder. The underbrush is extremely dense and consists of salmonberry, red elderberry, and devils club bushes. A few thimbleberry, hazel, red huckleberry and blackberry bushes grow on raised spots in the ravine. The ground is covered with grasses of several species, sword ferns, horsetail, and skunk cabbage. The floor of the ravine is cris-crossed with rotting fir, hemlock, and cedar logs that have fallen from the hillsides above. The soil of the hillsides is rocky glacial till, but the soil of the floor of the ravine is rich black humus. The stream begins at the western end of the ravine and flows eastward, gathering the water of many tiny springs. The stream disappears underground from time to time, to reappear a few feet further on, keeping the ground wet and marshy. The ravine described above is called area A. Two similar ravines, areas D and E, situated 4 and ? mile north, respectively, of area A, were used as control areas. Both are similar to area A, except that they are larger and have more coniferous timber and more and larger raised places. Both contain larger streams than area A, and neither is well bounded by hills. The flora and fauna are very similar to that of area A. While the small mammal population of area A fluctuated widely, that of the control areas was relatively stable.