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.