Staphylococcal Panton-Valentine leukocidin (PVL) is an important virulence factor, which causes leukocytolysis and tissue necrosis. Our previous report on the existence of the PVL genes (lukS-PV and lukF-PV) on the genome of prophage phiPVL in the Staphylococcus aureus strain V8 suggested the horizontal transmission of PVL genes by temperate bacteriophage among S. aureus (Kaneko, et al., 1998. Gene 215, 57-67). Here, we demonstrated the phage conversion of S. aureus leading to the production of PVL by discovery of a novel PVL-carrying phage, phiSLT (Staphylococcal Leukocytolytic Toxin) from a clinical isolate of S. aureus. phiSLT was able to lysogenize several clinical isolates of PVL-negative S. aureus strains as well as strain RN4220 at the conserved 29-bp sequence (attB) and all the lysogenized S. aureus strains had the ability to produce PVL. phiSLT had an elongated head of about 100x50 nm and a flexible tail of 400 nm long, that was quite different from phiPVL which had an isometric hexagonal head of about 60 nm diameter. The linear double-stranded phiSLT genome comprised 42,942 bp with 29-bp attachment core sequences and contained 62 open reading frames. Only 6.4 kbp region containing lysis cassette, PVL genes, attP, integrase, and orf204 of phiSLT was identical to that of phiPVL, while other regions were different from those of phiPVL. Thus, it can be concluded that PVL genes are carried by different temperate phages, which have the same attachment site.