A series of restriction-deficient mutants was isolated from non-lysogenic strains of Staphylococcus aureus belonging to phage groups I and II. Some mutants were sensitive to all phages tested. With one possible exception, all the mutants were unaffected in their modification systems. The breakdown of DNA of phages, restricted in the parental strains, was reduced in both the mutants that were tested. The restriction in propagating strain 3A could be transduced to its restriction-deficient mutant. The transduction efficiency increased after ultraviolet irradiation of the transducing phage suggesting that the gene for restriction is present on the bacterial chromosome.