We report a cluster of severe diarrheal disease caused by Vibrio mimicusinfection among four persons who had consumed leftover crayfish the day after a private crayfish boil. Gastrointestinal illness caused by Vibrio mimicushas not been reported previously in Washington State. Three cases were laboratory confirmed by stool culture; using PCR, isolates were found to have ctxgenes that encode cholera toxin (CT). Two of the cases were hospitalized under intensive care with a cholera-like illness. The illnesses were most likely caused by cross-contamination of cooked crayfish with uncooked crayfish; however, V. mimicuswas not isolated nor were CT genes detected by PCR in leftover samples of frozen crayfish. Clinicians should be aware that V. mimicuscan produce CT and that V. mimicusinfection can cause severe illness.