The "flat sour" organism, Bacillus thermoacidurans Berry (1933), is important to the canning industry because it is one of the few sporeforming organisms capable of growing in an acid food product, such as tomato juice. The seriousness of this type of spoilage lies in the fact that its occurrence is sporadic and as yet unpredictable. Furthermore, because little or no gas is produced to reduce the vacuum in the container, detection of the spoilage is impossible without first opening the container. The organism, a heat-resistant, sporeforming mesophile, was described by Berry (1933), who noted its tolerance to acid. In general, he observed that it produced only moderate growth in culture media and produced no gas in carbohydrate media, although acid was produced in a variety of sugars. Smith, Gordon, and Clark (1946) demonstrated from comparative cultural studies that Bacillus thermoacidurans Berry and the more recently described Bacillus dextrolacticus Anderson and Werkman (1940) are identical with Bacillus coagulans Hammer (1915), described some time before either of these names was proposed. The identity of these three organisms is also accepted by Breed, Murray, and Hitchens (1948). Although Bacillus coagulans is therefore the correct name for the species here discussed, so far as is shown by the studies made thus far, it has proved convenient to use the name Bacillus thermoacidurans in this paper for those cultures that were isolated from tomato products. The name Bacillus coagulans is used for the culture isolated by Hamer from evaporated milk. Although little work has been done on the physiology of Bacillus thermoacidurans, several papers of practical importance dealing with control measures designed to destroy the resistant spore have been presented. The organism is acidtolerant to a degree. Pederson and Becker (1949) found that although the vegetative cells of some strains could grow in tomato juice of pH 4.15 to 4.25 the heated spores could not germinate and grow in tomato juices that had been adjusted to a pH lower than 4.32. During the normal processing of juice the non-heat-resistant vegetative cells are readily destroyed. Further physiological studies have been undertaken to learn more of the nature of the organism in the hope of developing better measures for controlling its growth in foods. Cultures studies. The 23 cultures of Bacillus thermoacidurans and 1 culture of Bacillus coagulans studied included cultures isolated in the laboratory and others received from the laboratories of the National Canners' Association, Continental Can Company, American Can Company, and the American Type 1