Bacillus cereus is a recognized agent of food-borne disease. In this report we describe an outbreak of B. cereus gastroenteritis associated with consumption of beef stew among patients and staff at a Rhode Island nursing home. The beef had been improperly stored after preparation. The predominant symptoms of the illness were cramps and diarrhea; it lasted an average of 16 h. No deaths occurred. The organism was recovered from 10 of 23 stools collected from ill patients and 1 of 21 stools collected from controls (P = 0.0044, Fisher's two-tailed exact test). All isolates had the same biotype and serotype, newly designated H.26; all elaborated the diarrheal B. cereus enterotoxin when tested in rabbits by the vascular permeability reaction; and all had identical plasmid profiles, which differed from those of B. cereus strains selected randomly from other outbreaks. Plasmid analysis may prove to be a useful new tool in investigating outbreaks of B. cereus food poisoning.