This work is based on Dr. Washington Matthews studies of the Hidatsa Indians while stated at a military post and serving as a medical officer of the Army. This work presents what Matthews learned by observing manners, customs, and other characteristics, as well as making a close and careful study of their language. The original intent was to publish this treatise as a portion of a general work on Indian ethnography, but a delay in publishing the larger work made it desirable to prepare this material as a separate publication.