Concerning the S-R dissociation of Sh. sonnei there are abundant reports. The author obtained the following results by employing the virulent strain which had caused violent food poisoning. At first in order to facilitate the differntiation of S-R changes the Endo culture medium was partially modified and the variation of Sh. sonnei has been observed by performing the series of cultures on the medium taking serological aspects into consideration. 1) With the S-type the series of cultures successive incubations were carried out for 300 generations and always about five per cent of them indicated R-type dissociation. 2) In the series of cultures on the medium loaded with certain sugars the R-type dissociation became less apparent and after the fifth in every generation a wholly pure S-type was obtained. 3) In the experiments comparing the virulency of the bacteria cultured under the various conditions against animals, the virulency of the bacteria cultured in the soft gruel of rice malt proved to be strongest. 4) In the experiment of reconversion of the R-type bacteria to the S-type, either by culture in the media loaded with various substances or by passage through animals, no reconversion of R-type back to S-type has taken place at least in the author's experiments.