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Species that live in extreme conditions have specially adapted physiology and tissue/organ organisation. The adaptation of lymphoid organs to low temperatures in polar species could be an original field of study, indicating how the immune system works under extreme conditions. In fishes, the head kidney is a key organ for immunity and here the cytology of this organ is studied in two common Antarctic species: Trematomus bernacchii and Chionodraco hamatus. Ultrastructural analysis revealed heterogeneity of epithelial cells, with reticular cells, subcapsular- and perivascular-limiting cells. Differences in the size and morphology of epithelial cells were observed between the polar species and warm water species of fish. Intermingled with epithelial cell leucocytes, such as lymphocytes, thrombocytes and macrophages, had comparable morphology in both species, contrary to sharp differences observed in the morphology of erythrocytes and granulocytes. The functional adaptation of the head kidney to the low temperatures of polar water is discussed.