From one topotype and two specimens of _Castor canadensis repentinus_, from the Colorado River at Yuma, Yuma County, Arizona, _C. c. pallidus_ differs as follows: Tail and hind foot shorter. Color: Lighter throughout. Skull: Narrower; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 47 per cent); nasals barely projecting posteriorly beyond premaxillae; rostrum shorter; zygomatic breadth relative to basilar length less; mastoid breadth actually as well relatively greater; tympanic bullae narrower and smaller; coronoid process higher and wider; cheek teeth narrower. From one specimen of _Castor canadensis concisor_, from Trappers Lake, Garfield County, Colorado, and from the original description of that subspecies (Warren and Hall, 1939: 358), _C. c. pallidus_ differs as follows: Size smaller. Color: Markedly lighter throughout. Skull: Smaller, narrower; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 48 per cent); rostrum shorter; zygomatic breadth relative to basilar length less; mastoid breadth relative to zygomatic breadth greater; tympanic bullae narrower and smaller; jugals narrower; distal end of meatal tube smaller; coronoid process shorter and wider; angular process shorter and rounded rather than nearly pointed; cheek teeth narrower. From the type and near topotypes of _Castor canadensis rostralis_, _C. c. pallidus_ differs as follows: Size smaller; tail and hind foot shorter. Color: Markedly lighter throughout. Skull: Smaller and narrower; rostrum shallower and narrower; posterior end of nasals more constricted and barely projecting posteriorly beyond premaxillae; zygomatic breadth relative to basilar length less; mastoid breadth actually as well as relatively greater; dorsal surface of lacrimal bone larger; tympanic bullae narrower; coronoid process higher and wider; angular process not projecting so far caudad; cheek teeth narrower. From the type and near topotypes of _Castor canadensis duchesnei_, _C. c. pallidus_ differs as follows: Size smaller; tail and hind foot shorter. Color: Lighter throughout. Skull: Shorter, narrower and less massive; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 46 per cent); nasals barely projecting posteriorly beyond premaxillae; rostrum shorter and narrower; zygomatic breadth relative to basilar length less; mastoid breadth actually as well as relatively greater; tympanic bullae narrower and smaller; coronoid process higher and wider; angular process not projecting so far caudad; cheek teeth narrower.