Among known kinds of _Castor canadensis_, _C. c. rostralis_ is most like _Castor canadensis duchesnei_, from which the former subspecies differs as follows: Tail and hind foot longer. Color: Darker throughout. Skull: Nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 46 per cent); nasals less arched transversely; rostrum shorter, deeper and broader; ventral surface of rostrum less concave dorsally; dorsal surface of lacrimal bone smaller. For comparison with _Castor canadensis pallidus_, see account of that subspecies. _Remarks._--Animals from Kamas, in the drainage of the Weber River, are intergrades between _C. c. rostralis_ and _C. c. duchesnei_, but their short, wide nasals and wide rostra make them referable to _C. c. rostralis_. The available specimens of _C. c. rostralis_ are all from streams which ultimately empty into Great Salt Lake, which is in the northern part of the basin of Pleistocene Lake Bonneville. Some streams drain into the Lake Bonneville Basin without emptying into Great Salt Lake proper. Beavers from these streams, we suspect, when they become known, will be found to be related to _C. c. rostralis_. _Specimens examined._--Total, 16 (2 skins and skulls, 14 skulls only), distributed as follows: Summit County: Kamas, 5,500 ft., 6. _Salt Lake County_: Red Butte Canyon, Fort Douglas, 5,000 ft., 1; Millcreek Canyon, 6 mi. above mouth, 7,000 ft., 1. _Wasatch County_: Charleston, Heber Valley, 5,500 ft., 8. ~Castor canadensis duchesnei~ new subspecies _Type._--Male, young adult, skin and skull, number 4625, Museum of Zoology, University of Utah; Duchesne River, 10 miles northwest of Duchesne, 5,600 ft., Duchesne County, Utah; September 23, 1946; collected by Dave Thomas, original number 160 of K. R. Kelson. _Range._--Drainage of the Duchesne and White rivers in Utah and Colorado. _Diagnosis._--Size large; tail long (see measurements). Color (type): Upper parts Sayal Brown, purest on head, grading to Cinnamon Buff at base of tail; underfur Fuscous; hind feet Burnt Umber; ears Fuscous Black; underparts Tawny Olive; underfur Smoke Gray. Skull: Large, massive; nasals long, slender (breadth averaging 46 per cent of length) and markedly convex transversely; rostrum long and attenuate; zygomatic arches heavy and widely spreading (zygomatic breadth averaging 81.5 per cent of basilar length); ventral surface of rostrum markedly concave dorsally, especially immediately behind upper incisors; nasals extend posterior to premaxillae.