Three colonial types of S. sonnei are described which were associated with corresponding changes in antigenic structure of the organisms. Phase I culture appeared to correspond to the smooth of previous descriptions. Phase II was distinct in colony appearance and agglutination reactions, but was not a rough variant. Both phases I and II were found occurring naturally in cultures of S. sonnei and in stool cultures from cases of human S. sonnei infection but most cultures upon repeated transfer stabilized in phase II. A third colony type, R, (which had the usual characteristics of a true rough variant) was obtained from old cultures. This showed some, but slight, serological relationship to the phase II variant. The analogy between the phase I-phase II variation and the specific-group variation which has been described for certain paradysentery organisms is pointed out. The importance of the I–II variation in the serological identification of S. sonnei is discussed and attention is called to the selective action of certain differential media which permit growth of phase I forms while repressing phase II and colony type R.