The ultrastructure of the main lymphomyeloid organs of the holocephalan fish, Chimaera monstrosa, is presented. The thymus is well developed even in adults. It is densely packed with small and large lymphocytes, the former predominating. The ultrastructure of C. monstrosa's lymphocytes is similar to that of mammalian ones. The lymphocytes show intimate connections with large epithelial reticulocytes, which infiltrate the cytoplasm of the lymphocytes with long processes. The lymphomyeloid tissues around the orbit are mainly granulocytopoietic. Two types of granulocytes occur: eosinophilic and heterophilic. The latter is found in excess. The granulocytes are ultrastructurally similar to those of elasmobranchs such as Etmopterus spinax and have similar characteristic granules. In addition to granulocytes in various stages, lymphocytes, plasma cells, blast cells, and solitary macrophages are scattered throughout the tissue. The suprapalatal region also has a similar supply and variety of cells, but most heterophilic granulocytes have modified granules. In the spleen, erythrocytes in different developmental stages are common. This confirms earlier studies which suggested that the spleen functions primarily as an erythrocytopoietic organ.