Antagonistic phage-bacteria interaction gives rise to the set of host’s resistant clones that differs in ability to support the reproduction of phage h-mutants. This relationship determines their co-evolution, expressed in coined term – ‘evolutionary arms race’ of antagonists. The goal of the research was to investigate a co-evolution of wild type phage 8573 sm and phytopathogenic strain P.fluorescens. To achive that we used Pseudomonas fluorescens IMV 8573 and wild type phage 8573 sm, isolated from the natural environment. Subsequently we obtained the resistant clones of P. fluorescens and phage h-mutants that were able to overcome bacterial resistance barrier. We expected the resistant subpopulations of bacteria infected by phage 8573 sm and phage h-mutants to be similar in their phenotypic characteristics. However, isolated phage mutants differed for the type of their plaques. Our studies have shown that interaction of each resistant host clone with phage 8573 sm determines the selection of highly specific phage mutants. According to the results the initial members of the co-evolutionary process (phage-bacteria) had only one genotype. However, as a result of their coevolutionary interaction a number of various phage and its host mutants emerged. Intriguingly, they were not able to cross infect other resistant host clones except thier own. The genome comparison of original phage 8573 sm and mutants was performed using restriction analysis with endonuclease Bgl II and Hind III. It was found that mutant genomes had different number of the restriction sites. The obtained data provide an interest for understanding of existing factors that limit the number of bacteria in the nature, and provide a possibility to detect the selection of antagonists during their interaction, which is an evolutionary adaptation that keeps them from total destruction.