Caliciviruses have a geographically and phylogenetically diverse host range, and are responsible for significant disease outbreaks in many animal species including livestock, pets, marine mammals, and humans. Many of the known caliciviruses have an ocean origin, with a large number of isolations coming from marine mammals. Researchers, however, suspected that marine mammal populations may not be the primary reservoir for caliciviruses of ocean-origin, and a calicivirus was subsequently isolated from fish (Girella nigricans). The marine environment encompasses a larger variety of potential host organisms other than marine mammals and fish. For example, caliciviruses have been isolated from invertebrate species and have been implicated in human gastroenteritis associated with the ingestion of contaminated shellfish. Twenty-two mussels (Mytilus californianus) were collected from the rocky intertidal zone of Point Bennett, San Miguel Island in the Spring of 1992. Gill and intestinal tissue, as well as residual water, from the shellfish were processed separately for virus isolation and calicivirus cDNA hybridization. A calicivirus was isolated from one mussel intestinal sample, and was identified by serum neutralization assay as a strain of San Miguel sea lion virus, type 17 (SMSV-17), first isolated on San Miguel Island, in the Spring of 1991, from the nasal swab of a dead, premature sea lion pup (Zalophus californianus). Negative stain electron microscopic examination of the original mussel tissue sample revealed recognizable calicivirus particles with smudged capsid morphology compared to calicivirus particles passed in cell culture. Of the 66 samples assayed with the calicivirus cDNA hybridization probe, 59 were positive, including the intestinal sample from which SMSV-17 was isolated. These findings suggest that many of the caliciviruses in the marine environment may be difficult or impossible to cultivate. Mussels collected from Port Angeles, Washington, and Yachats, Oregon, regions not associated with high densities of pinnipeds, were all negative when assayed with the hybridization probe. Mussels and possibly other bivalve mollusks appear to be an important ocean reservoir for caliciviral agents.