In November 1993, clusters of gastroenteritis in six states following oyster consumption were investigated to identify common features, and stool samples were obtained to identify a pathogen. Efforts were made to account for all potentially contaminated oysters using harvest tags and the interstate recall system. Consumption of oysters was associated with illness in 10 clusters; no other food was implicated. A Norwalk-like virus was detected by electron microscopy in 9 of 18 samples and by reverse transcription-polymerase chain reaction in 20 of 26 samples from 6 clusters. Nucleotide sequences of a 123-bp fragment from all specimens were identical, consistent with a common source outbreak. Implicated oysters were harvested from the Louisiana coast between 9 and 12 November. Although some were recalled and destroyed, most oysters harvested from the area during this time remain unaccounted for. Current regulations and commercial practices need to be revised to permit thorough tracing and recall of contaminated oysters and to improve control of future epidemics.