The Pacific hagfish, Eptatretus stoutii, is capable of a variety of immunologic responses including allograft rejection and serum antibody production to soluble and cellular antigens. Interest has revived in the morphology of hagfish lymphoid tissues. The search for a thymus in young specimens has resulted in the discovery of a phagocytic and antigen-receptive cell population associated with the pharyngeal velar muscles. We suggest that a protothymus or precursor of the thymus of higher vertebrates may be contained within this velar muscle complex.