Fossil mammals are described from two units of a distal facies of the volcanogenic Ohanapecosh Formation in the Tieton River area, 30 miles southeast of Mount Rainier. A tooth fragment of a rhinocerotid or hyracodontid from the Milk Creek tuff is suggestive of a late Chadronian to Orellan age indicated by the published zircon fission track age of 34 Ma. This age places the fossil as the oldest terrestrial mammal recovered from Washington. Isolated elements collected from the overlying upper Wildcat Creek beds belong to Cormocyon copei, Enhydrocyon sp., Parenhydrocyon josephi, a cricetid rodent, cf. Palaeolagus sp., Miohippus equinanus, Miohippus equiceps, Diceratherium annectens, Promerycochoerus superbus, Eporeodon sp., Merycoides sp., Mesoreodon sp., Hypertragulus sp., and a tayassuid. These taxa, the Wildcat Creek local fauna, show a northern extension of the middle John Day fauna of eastern Oregon. Their occurrence in Oregon suggests a late early Arikareean (26-28 Ma) age. This fauna lived in a moist seasonal environment, indicated by a Goshen-type paleoflora in the lower Ohanapecosh Formation and supported by a succession of Argillosols and Gleysols to gleyed Protosols.