Staphylococci adapt specifically to various animal hosts by genetically determined mechanisms that are not well understood. One such adaptation involves the ability to coagulate host plasma, by which strains isolated from ruminants or horses can be differentiated from closely related human strains. Here, we report first that this differential coagulation activity is due to animal-specific alleles of the von Willebrand factor-binding protein (vWbp) gene, vwb, and second that these vwb alleles are carried by highly mobile pathogenicity islands, SaPIs. Although all Staphylococcus aureus possess chromosomal vwb as well as coagulase (coa) genes, neither confers species-specific coagulation activity; however, the SaPI-coded vWbps possess a unique N-terminal region specific for the activation of ruminant and equine prothrombin. vWbp-encoding SaPIs are widely distributed among S. aureus strains infecting ruminant or equine hosts, and we have identified and characterized four of these, SaPIbov4, SaPIbov5, SaPIeq1 and SaPIov2, which encode vWbpSbo4, vWbpSbo5, vWbpSeq1 and vWbpSov2 respectively. Moreover, the SaPI-carried vwb genes are regulated differently from the chromosomal vwb genes of the same strains. We suggest that the SaPI-encoded vWbps may represent an important host adaptation mechanism for S. aureus pathogenicity, and therefore that acquisition of vWbp-encoding SaPIs may be determinative for animal specificity.