Use of selection method with R. Ffm phage offered a possibility of obtaining R-forms of Sh. sonnei from the population of bacteria of phase II with a low frequency of natural dissociation. Mono- and polyauxotrophic mutant of I, II and R-phases by hir, ilv, pro, and mtl markers were obtained by treatment of Sh. sonnei strains with nitrosoguanidine and ethyl-methanesulfonate. The frequency of mutation in two Sh. sonnei strains studied in this work (UBC and No. 941) by the same genetic regions proved to differ. The change of Sh. sonnei phase I into phase II was observed only in selection by proline marker which could be connected with the existence in this region of a gene controlling the dissociation process.