Shellfish are known to accumulate and retain microbial pathogens contained in the water column. Depuration and/or relaying work well for removing bacterial pathogens and perhaps some pathogenic viruses. Norwalk-like caliciviruses, however, have been implicated in major outbreaks of gastroenteritis where raw shellfish were consumed and where the shellfish had been cultivated, harvested, and processed such that other pathogenic microbes were absent. Because caliciviruses are present in ocean reservoirs and because we had previously isolated a putative pathogenic calicivirus from naturally exposed edible shellfish, we began to investigate the effect of depuration on calicivirus retention in experimentally exposed clams (Mya arenaria), oysters (Crassostrea virginica), and mussels (Mytilus californianus) with the secondary objective of determining whether certain Norwalk-like caliciviruses could replicate in shellfish. Groups of each shellfish species were exposed to SMSV-17 by three routes: 1) direct inoculation into the hemolymph, 2) virus placed directly into sea water holding tanks, and 3) feeding algae mixed with the virus. Exposed shellfish were held in tanks having a flow-through system of purified, UV treated sea water. Five shellfish of each species were assayed at times 0, 0.5, 1, 2, and 8 hours and 1, 2, 4, 8, 16, 30, and 60 days. Individual tissues (gut, reproductive tract, mantle, gill, and hemolymph) were processed for virus isolation. Under laboratory conditions of holding which were more rigorous than commercial depuration, SMSV-17 was isolated up to 60 days post-exposure, in a mammalian cell line incubated at 37°C, from a variety of tissues of the three bivalve mollusk species. This suggests that caliciviruses pathogenic for mammals can be maintained in shellfish reservoirs for long periods of time, even under conditions of continuous depuration.