Twenty-eight different species of the genus Bacillus have been examined both as to morphological characteristics and physiological reactions. The physiological reactions tested include gelatin liquefactions, acetyl-methyl-carbinol production, nitrate reduction and 36 different carbohydrate fermentations. The action of all 78 strains of the 28 species was also determined on peptone and graphs have been prepared comparing this action with that of the organisms of the carbohydrates. Definite explanations are given for many of the inconsistencies existing in the literature for the fermentation reactions of the bacilli. The results obtained are compared with those recorded by three previous workers, namely Fitzgerald (8), Soriano (29) and Bergey’s Manual (1). Reasons have been advanced to explain cases of disagreement. These are concerned mainly with the indicator used and the failure to take into account the action of the organisms under investigation upon the basal medium. Where it was felt that a representative number of strains were examined, definite conclusions as to the ability of the organisms to dissimilate various carbohydrates are drawn. A key for the classification of the B. mesentericus group has been formulated based entirely on carbohydrate fermentations. The procedure followed appears to be applicable to other groups as well as to the entire genus. / Master of Science