<jats:p> The synthesis of virulence factors and other extracellular proteins by <jats:italic>Staphylococcus aureus</jats:italic> is globally controlled by the <jats:italic>agr</jats:italic> locus, which encodes a two-component signaling pathway whose activating ligand is an <jats:italic>agr</jats:italic> -encoded autoinducing peptide. The cognate peptides produced by some strains inhibit the expression of <jats:italic>agr</jats:italic> in other strains, and the amino acid sequences of peptide and receptor are markedly different between such strains, suggesting a hypervariability-generating mechanism. Cross-inhibition of gene expression represents a type of bacterial interference that could be correlated with the ability of one strain to exclude others from infection or colonization sites, or both.