Staphylococcus aureus strains 7-8 and 57 that produce beta-hemolysin but not staphylokinase (beta + K-) were lysogenically converted by certain serological group F bacteriophages to the loss of beta-hemolysin production and the gain in staphylokinase production (beta-K+). Serological group A phage 42E was found to convert S. aureus strains 7-8(beta-K-) and 57 (beta + K-) to beta - K-. Conversion of beta-hemolysin by lysogenization of a serological group A phage has not previously been reported. Phage 42E conversions differed from the group F conversions since staphylokinase was not affected. This indicates that conversion to beta-K+ involves separate loci on the phage chromosome. Several characteristics associated with virulence of staphylococci of human or animal origin other than staju;plomase production (coagulase, DNase, lipase, gelatinase, mannitol fermentation, and phage-sensitivity patterns) were not correlated with lysogenic conversions to loss of beta-hemolysin.