Seven bacteriophages specifically lytic for representative strains of Shigella flexneri serotype 6 were studied. Cross hemagglutination experiments and chemical analysis of isolated lipopolysaccharides showed that strains 975 and 323 are smooth and strains 288, 551 and 488--rough. Lipopolysaccharides isolated from the smooth strains inactivated phages F10 and "a" whereas those from the rough strains demonstrated poor inactivation of specific bacteriophages. The lipopolysaccharides of both the smooth and rough strains were found to be phage "a" receptors. The results indicate that the varied sensitivity of S. flexneri 6 depends on the roughness of the strains.