While the etiology of amoebic dysentery was thoroughly investigated and its connection with amoebae fairly well established during the decade from 1880 to 1890 it was not until 1898 that Shiga isolated the causative organism of bacillary dysentery. It is true that Chantamesse and Widal drew attention to a bacillus isolated from large intestines, mesenteric glands and spleen of cases of tropical dysentery but the organism was not clearly differentiated from _Bacillus coli_. Celli isolated an organism which coagulated milk and produced gas in glucose media. This organism which Celli called _B. coli dysentericus_, differs culturally from _B. dysenteriae_. =Geographical Distribution.=—Bacillary dysentery differs from the amoebic form in that it tends to appear in extensive epidemics spreading over temperate as well as tropical and subtropical parts of the world. It is peculiarly liable to follow the movements of armies in any part of the world and like typhoid fever its distribution is one of hygienic rather than geographical influence. Infections with various strains of dysentery bacilli are important factors in morbidity among infants and young children in whatever part of the world the question has been investigated. The disease is prone to prevail in lunatic asylums whether in temperate or tropical parts of the world. ETIOLOGY AND EPIDEMIOLOGY =Etiology.=—During a very fatal epidemic of dysentery in Japan Shiga isolated an organism, _Bacillus dysenteriae_, from dysenteric stools of 36 cases, which bacillus he found to be agglutinated by the serum of the patients. He reported this work in 1898. In 1900, Kruse isolated an organism from patients in an epidemic of dysentery in Germany which corresponded to that of Shiga. In 1900, Flexner, Strong and Musgrave, working in Manila, not only encountered an organism similar to that of Shiga but also an organism of wider fermentative action. Dysentery has resulted from accidental laboratory infections and Strong produced dysentery in a prisoner condemned to death through ingestion of cultures. In 1903, Hiss and Russell isolated an organism from a fatal case of diarrhoea in a child to which they gave the name “Y”. On the whole, dysentery bacilli correspond culturally with the typhoid bacillus except in showing slightly weaker fermentative action on carbohydrates. The main point of difference however is their absolute nonmotility.