S.flexneri 2a strains isolated in 1992-1994 from 371 patients in sporadic cases, from 90 patients and their contacts in cases of group diseases and from 71 lethal cases of Flexner's dysentery, were studied in comparison with strains isolated in 1980s. The study revealed the biological heterogeneity of S.flexneri 2a strains. The circulation of 3 enzymatic variants and 11 variants differing in their sensitivity to colicins from Frederick's collection was established. The overwhelming majority of strains circulating in 1990s were resistant to 6-7 antibiotics, while the strains isolated in 1980s were resistant only to 4 of these antibiotics. The strains under study possessed antilysozyme activity, as well as pronounced invasive properties. The circulation of the population of multiresistant strains in the city could affect the course of present-day Flexner's dysentery and their domination in the ecological structure of Shigella infections.