As a result of the comparative study of Leach1, the blackleg disease of potatoes is now generally ascribed to Bacterium carotovorum, which is also responsible for the soft rots of many of our vegetables and other plants. Previous investigators2,3,4,5 had described and named slightly different organisms which Leach endeavoured to show were either-identical with or at most variants of B. carotovorum. This conclusion was based mainly upon four sets of observations recorded by himself or by others: (1) the similarity of the reactions in dextrose, sucrose and lactose; (2) the proved pathogenesis of authentic strains of B. carotovorum when inoculated into potato stems; (3) the isolation of this organism from a soft rotted tuber; (4) the blackening of "potato tissue killed by almost any organism" (Leach).