The antimicrobial susceptibilities of 572 strains of Escherichia coli were evaluated. The strains were divided according to the biotypes and the differences between the strains towards various antibiotics were examined. We found a correlation between biotype and antibiotype. This correlation lead us to think of a possible existence of more resistant biotypes than others. Now we are doing other studies in order to show if this correlation is constant during time and if similar differences are present for other microorganisms, as well.