From November 1972 to the end of 1975, Shigella sonnei types implicated in dysentery morbidity in a number of districts of East Bohemia region were studied. 301 strains subjected to phage typing and colicine typing were classified into 45 "combined" types. During the study five types prevailed in succession, each for a protracted period of time. Four of these types retained, in the great majority of cases, their original high drug sensitivity throughout the period of their prevalence. Other types were only encountered transiently and sporadically. As a result of a phagerestrictive effect of newly acquired plasmids there occurred, in rare instances in some strains, a change in phage type; or the colicine type changed while the phage type was retained as a result of another col factor having been acquired by the strain. The changes occurred in the course of epidemics.