There are just as many reports on the l-histidine decarboxylase activity of dysentery bacilli as those on the other bacteria. In the course of elucidation of the causative factors of violent food poisoning that occurred in the Okayama Prefecture in 1953, the author has made a report on the S-R dissociation and the virulency of Sh. sonnei in the Report 1. In the preaent report the biological nature of the virulent strain is described. 1) Sh. sonnei grow most rapidly in the medium whose optimal pH is 7.0-7.6, but when the pH falls under 4.3 the growth of bacteris is completely inhibited: and in addition, the peaks of the growth are correlated with changes in the pH of the medium. 2) In the cultures performed in test-tubes and flasks, R-type bacteia grew in the flask much better than S-type, while on the contrary in the test-tubes R-type bacteria growth was inferior to that of the S-type. 3) As for the growth of Sh. sonnei in soft gruel of rice malt, although the growth does not reach as high degree as observed in the peptone medium but it reaches the maximum growth within 12 hours. After 24-hour culture, however, the growth tends to decline. 4) On examining by paper chromatography and by Warburg's apparatus the l-histidine decarboxylase activity of the isolated strain that has been cultured in the media of peptone solution loaded with l-histidine and with additional various sugars, the excretion of carbon dioxide gas and the production of histamine are observable only in the cases where these bacteria are grown in the media containing additional glucose, maltose, or mannose. 5) Acetone powder of Sh. sonnei that has been cultured in the medium of glucose-peptone solution loaded with l-histidine possesses the l-histidine decarboxylase activity and acts specifically to the substrate.