When foreign materials enter the blood stream of crustaceans, hemocytes bind to other hemocytes and pathogens to form nodules. These rapidly increase in size until they occlude small vessels, most numerous in the gills. Although this method for clearing foreign particles from circulation is effective, it causes a significant decline in number of circulating hemocytes. The goal of my study is to learn where the hemocyte-forming tissue is located, what hemocyte stem cells look like and what are early stages in hemocyte maturation. The first step is to determine location and morphology of the hematopoetic tissue using light and scanning electron microscopy. I found it as nodules in a thin tissue between the foregut and epithelium near the dorsal surface of the head. To stimulate mitosis in hematopoietic tissue, hemocytes were removed from circulation by injecting the ridgeback shrimp, Sicyonia ingentis , with 0.1mL of 106 bacteria, Bacillus subtilis . Hemocyte counts (cells/mL) were determined for all shrimp at the start of each experiment.Then every other day for two weeks, two shrimp were selected for a second hemocyte count and removal of hematopoietic tissue. Sections through the tissue should allow us to determine the mitotic index (#dividing cells/#cells), which should peak before circulating hemocyte levels return to pre-injection levels. Eventually, dividing cells in this tissue will be labeled with a marker that inserts into the DNA and is retained when released into circulation. We plan to use this method to determine the lifespan of circulating hemocytes.