Recent concerns regarding the safety of shellfish consumption have focused on the risk posed by naturally occurring marine bacteria such as Vibrio species and by viruses such as Norwalk and related agents. Despite the widespread environmental presence of Vibrio species in the Chesapeake Bay, the rate of reported infections remains low; there have also been no reports of major Norwalk outbreaks associated with shellfish in this area. As infections with these agents may not always be recognized because of difficulties in making the diagnosis and/or their mild or subclinical presentation, a serosurvey was conducted among healthy volunteers living in the Chesapeake Bay region. Serum and questionnaire data were collected during the fall of 1987 from 267 persons with varying levels of exposure to shellfish: shellfish industry workers, persons attending a local seafood festival, and Seventh-day Adventists (who traditionally abstain from eating shellfish). In comparisons among groups, a significant association could not be demonstrated between shellfish consumption or contact and antibody response to Vibrio cholerae O1 or Norwalk virus. Rates of seropositivity were high for both agents (up to 22% seropositive with a V. cholerae O1 Inaba vibriocidal assay, 14% with an enzyme-linked immunosorbent assay for cholera toxin, and up to 70% seropositive with an enzyme-linked immunosorbent assay for antibodies to Norwalk virus); the basis for these responses in population-based studies remains to be determined. Shellfish industry workers did have a significantly elevated antibody response to the unencapsulated phase variant of Vibrio vulnificus as compared with the other groups studied. Infection with V. vulnificus may be relatively common among persons with high levels of exposure to shellfish.