An outbreak of Salmonella Thompson infections occurred among persons who ate food prepared by a restaurant in Sioux Falls, South Dakota. The epidemiologic investigation implicated the roast beef. Salmonella Thompson was isolated from leftover roast beef and from ham. The roast beef was more heavily contaminated with Salmonella than was the ham. The roast beef appeared undercooked. There had been problems with temperature holding of the meats. Allowing the meat to stand at room temperature for several hours would allow Salmonella bacteria to multiply.