The polyvalent staphylococcal bacteriophage phi812 and its host-range mutants, members of Myoviridae family, lyse most of Staphylococcus aureus strains and are, therefore, used in therapy of staphylococcal infections. However, some 5-10% of the strains are insensitive to these phages, which may reduce the effectiveness of phage-based therapy. In this work we analysed the possible defence mechanisms of the strains which can result in their insensitivity to phages. Therefore, we examined correlation between strain insensitivity and efficiency of phage adsorption on bacterial cells, presence of the SauI restriction-modification (RM) system and content of resident prophages. For the study, a set of genotypically well characterized S. aureus strains from distinct clonal complexes (CC) differing in their prophage content was used.