Three-dimensional reconstructions of a hematopoietic organ (HPO) from Bombyx mori larva were undertaken using light and electron microscopy. Each compact islet varied in sizes, but in the central area of the HPO their size became smaller. Compact islets and loose islets were made up of prohemocytes, plasmatocytes, and reticular cells, but there were differences in the proportions of these cells. Within the cytoplasm of reticular cells and within their cell projection, vacuoles were observed. Cell proliferation occurs primarily in the compact islets, and differentiation associated with the reticular cells occurs primarily in the loose islets. It can be inferred that reticular cells have a significant influence on proliferation and differentiation associated with hematopoiesis. According to the results of the 3-D reconstruction, one reticular cell is in contact with eight or nine hemocytes. Each reticular cell is presumably of approximately ten hemocytes. Movies relevant to Figs. 3, 4 and 5 can be found at http://biotech.korea.ac.kr/$~$cellbio/pages/3D-results.html.