Recently discovered fossil material from the late Arikareean of western Nebraska is referable to a new genus and species of castorid, Priusaulax browni. This species is the earliest record of a castoroidine beaver in North America. The skull possesses a combination of derived castoroidine features (divergent tooth rows, loss of stapedial foramen, premolars larger than molars, grooved palate) and primitive agnotocastorine features (elongated rostrum, procumbent incisors, contact between lachrymal and jugal bones). Morphologically and temporally, Priusaulax is transitional between the earlier agnotocastorines with more primitive characters and the later castoroidines with more derived characters.