Our long-term goal is to understand how crustaceans fight infections and propose methods to prevent disease in aquaculture settings. Shrimp, Sicyonia ingentis , were collected off the coast of Palos Verdes and used in two studies this summer. First, to simulate bacterial infection, 10<sup>6</sup> bacteria were injected into individual shrimp. Circulating blood cells respond by becoming sticky and clustering with bacteria. Although this process eliminates circulating bacteria, hemocytes decrease also. Following bacterial injections, we took blood cell counts and samples of hematopoietic tissue every 24hrs for two weeks to determine the rate at which the blood cell level returned to normal and to follow morphological changes in maturation of developing blood cells. Our second topic concerns morphology and biochemical composition of the midgut trunk (MGT). This is the only surface not covered by an exoskeleton and seems likely for bacterial penetration. The composition of the extracellular matrix of the MGT, the unusually thick basal lamina and peritrophic membrane were studied by digestions and SDS-PAGE analysis. We are also studying permeability of the intact MGT and isolated acellular layers by following the ability of different sized tracers (lanthanum, ferritin, and latex beads) to penetrate the various layers of the MGT. These projects will aid our understanding of how bacteria can gain entrance into the shrimp blood system and the time it takes for the normal level of circulating blood cells to be re-established after the initial infection is cleared from the blood.