The objective was to use polymerase chain reaction (PCR) and nucleic acid-based transcription and hybridization technologies to detect and follow the course of Norwalk virus (NV) progression within shellfish after bioaccumulation from environmental sea water.  The specific goals were to determine the tissue distribution of NV in oysters and clams and to determine whether tissue-associated viruses adversely affect shellfish depuration effectiveness.  Long-term, the project is expected to lead the exploration of practices that will enhance depuration effectiveness by determining which practices promote elimination of tissue-associated viruses. The eastern oyster, Crassostrea virginica, and the hard shell clam, Mercenaria mercenaria, were exposed to varying concentrations of NV for 4-24 hour bioaccumulation times, and tissues including the stomach, digestive diverticulum, adductor muscle, and hemolymph cells were examined for the presence of virus, using RT-PCR, in situ transcription, and in situ PCR techniques. Duration effectiveness was compared to a bacterial indicator standard, E. coli, in experiments with C. virginica.  NV was present in all oyster and hardshell clam tissues examined following bioaccumulation. NV was depurated less efficiently than E. coli from oysters.