The object of the present investigation was to determine quantitatively the extent of glucose and d- and l-arabinose utilization in order to separate it from the other two factors influencing the pH change in the medium. The pH of the carbohydrate media employed was determined potentiometrically, using a glass electrode in order that the pH produced could be compared with the extent of utilization of these carbohydrates. Likewise, the pH produced by the same <i>Bacilli</i> in peptone alone was determined so that it could be compared with the pH produced in the carbohydrate-peptone media. The fundamental observations concerned with the metabolism of the genus <i>Bacillus</i> (which may be derived from this study) may be of value in the taxonomic identification of various members of this group of organisms, and might lead to other interesting conclusions regarding the metabolism of these organisms in pure laboratory culture and also in mixed cultures in the natural habitat. / Ph. D.