The fine structure of the hemopoietic tissue and its detailed reticular organization in the mealworm beetle, T. molitor were examined using light and scanning electron microscopes. The major hemopoietic tissues in the abdomen were located on the upper surface of the dorsal diaphragm which continuous over the ventral wall of the heart. Histologic characteristics of this hemopoietic tissues are dense clusters of cells. They are irregular in outline and are not surrounded by any connective tissue sheath. The hemopoietic tissue of this insect is consisted of three cellular components which are the reticular cells, hemocytic stem cells and several kinds of mature hemocytes. The reticular cells had numerous cytoplasmic processes and forming a complex network. The stem cells give rise to differentiating hemocytes of the different cell lineages. Mature hemocytes within this hemopoietic tissue are originated from the stem cells and differentiated into several types of hemocytes including prohemocytes, plasmatocytes, and granulocytes.