The thymus of the rhesus monkey ( Macaca mulatta) was studied by histological, histochemical and transmission electron microscopic techniques. The results showed that it was surrounded by a capsule continuous with thin connective tissue septa that divided the organ into many lobules, each consisting of cortex and medulla, and most of the medullae connected with each other. There were many mast cells in the capsule and interlobular septa. The cortex mainly consisted of immature T lymphocytes, epithelial cells, some mast cells, and macrophages and, in addition, typical blood-thymus barrier. The medulla was mainly composed of mature T lymphocytes, epithelial cells, also some mast cells and a few macrophages, interdigitating cells, myroid cells, B lymphocytes, plasma cells, eosinophilic granulocytes, thymic corpuscles and thymic cysts.