In late October 1991, an outbreak of gastroenteritis, following the consumption of raw oysters involving more than 200 people, was reported in five locations in Quebec, Canada. Bacteriological analysis of the oysters involved indicated low levels of fecal coliforms, but direct electron microscopy of stool samples obtained from two people involved in the outbreak revealed that both contained 27–34 nm, small, round Norwalk-like viruses. Immunoelectron microscopy, using acute sera obtained from these individuals and convalescent serum from another person related to the outbreak, revealed antibody coatings on these Norwalk-like viruses with all three sera. Solid-phase immunoelectron microscopy demonstrated that these viruses were also antigenically similar or related to a Norwalk-like virus isolated as the cause of a gastroenteritis outbreak in a home for the aged between December 1988 and January 1989 in Thunder Bay, Ontario. From these findings and the symptoms of the illness, the Norwalk-like virus was considered as the causal agent of the outbreak due to the consumption of contaminated oysters. How the oysters became contaminated was not determined. Oysters harvested from the areas initially thought to have been the origin of the implicated shellfish were tested for the presence of viral fecal indicators using tissue culture and electron microscopy with negative results. It is most likely that the implicated lot also contained oysters harvested from another area, also open, but which was downstream from an identified source of human fecal contamination.