The genome of Staphylococcus aureus strain S6C was shown to contain a prophage inserted within the beta-toxin (BT)-encoding structural gene (hlb). The phage att site was identical to that reported for the BT-converting phages phi 13 and phi 42. The prophage carried the genes encoding staphylokinase (sak) and enterotoxin A (sea), which suggests that it is similar to phi 42. However, it was not included in the presence of mitomycin C (MC) and appears to be defective. Mapping studies revealed that the genomes of the BT-converting phages present in strains S6C and PS42D (a phi 42 lysogen) encode at least one SmaI restriction site. Moreover, the PS42D chromosome contained a second prophage that also had at least one SmaI site, carried both sak and sea, and hybridized with DNA probes that also hybridize with the BT-converting phages. The second phage in strain PS42D was mapped to a SmaI fragment corresponding to fragment A of the S. aureus strain 8325 genomic map. Although the BT-converting phage present in strain S6C could not be induced, a phage was induced from strain S6C using MC. Southern blots suggest that is is similar to phi 11; however, the restriction patterns of DNA from the induced phage and phi 11 were clearly distinct. We have designated the inducible phage present in strain S6C as phi 15, to denote the distinction. Relatively weak hybridization signals were also observed when phi 15 DNA was used to probe genomic DNA from S. aureus strains lysogenized with the BT-converting phages, phi 13, phi 42 and 42E.(ABSTRACT TRUNCATED AT 250 WORDS)