1. The several standard cultures used in this study are indistinguishable —a conclusion previously reached and stated by Flexner. 2. The acute dysentery of the United States is due to a bacillus indistinguishable from that obtained from the epidemics of dysentery in several other parts of the world. 3. The sporadic and the institutional outbreaks of acute dysentery are caused by the same microorganism, and this organism is identical with that causing epidemic acute dysentery. 4. The cause of acute dysentery, whether sporadic, institutional, or epidemic, is Bacillus dysenteriæ Shiga.