Tamai, Kenzo (Kanazawa University, Kanazawa, Japan), and Shoki Nishida. Taxonomy of Clostridium bifermentans and Clostridium sordellii. II. Toxigenic and sporulating potencies in substrains of a Clostridium sordellii strain. J. Bacteriol. 88:1647-1651. 1964.-The existence of six biological criteria for distinction of Clostridium bifermentans and C. sordellii was confirmed. The difference in the six criteria gradually disappeared as the sporulating potency of the substrains of C. sordellii 4708 was strengthened. The substrains which could resist heating at 90 C for 10, 20, or 30 min were found to have lost all six criteria for distinction and were biologically in agreement with C. bifermentans. We further demonstrated that all newly isolated strains of C. bifermentans examined possessed extremely strong sporulating potency.