The development of the thymus was examined in different stages of Harpagifer sp. from Signy Island (South Orkney Islands; 60°43′S, 45°38′W). The thymus was typical, both in position and structural development, of that observed in warmer-water teleosts. The infiltration of the thymic epithelia was not observed until 4 weeks post-hatch. Full development of the lymphoid organs was not achieved until the juvenile stage. Although an increased infiltration of the thymus, by sub-epithelial connective tissues and epithelial mucous cells, occurred in the juvenile and adult stages, there was no evidence of an advanced stage of thymic regression or involution in the adult Harpagifer. Thus a suppressive influence of the low temperature environment, on the onset and degree of thymic development and involution, was indicated in this species.