Coagulase-negative staphylococci (CoNS) are recognized as significant opportunistic pathogens. However, current knowledge of virulence mechanisms is very limited because a significant proportion of CoNS are refractory to available techniques for DNA transformation. We describe an efficient protocol for plasmid transfer using bacteriophage Φ187, which can transduce plasmid DNA to a wide range of CoNS from a unique, engineered Staphylococcus aureus strain. The use of a restriction-deficient, modification-proficient S. aureus PS187 mutant, which has a CoNS-type bacteriophage surface receptor, allows plasmid transfer to CoNS even when they are refractory to electroporation. Once the Φ187 titer reaches 10(9) plaque-forming units per milliliter, plasmid transfer can be accomplished within 1-2 d. Thus, our protocol is a major technical advance offering attractive opportunities for research on CoNS-mediated infections.