Bacteriocins are proteinaceous compounds with bactericidal activity against other bacteria. Our laboratory has been investigating bacteriocin production by Staphylococcus sp. strains from different sources. Two bacteriocins produced by S. aureus (aureocins) were characterized at the molecular and biochemical levels. Aureocin A53 is an atypical class II bacteriocin produced by S. aureus A53 isolated from commercial milk. This antimicrobial peptide is encoded by a 10.4–kb plasmid (pRJ9) and is able to inhibit several Gram-positive bacteria, including Listeria monocytogenes, streptococcal strains involved in bovine mastitis and staphylococcal strains associated to human infections and bovine mastitis. Aureocin A53 is a highly cationic 51-residue, non-processed peptide. Further unique features of aureocin A53 include remarkable protease stability and a defined, rigid structure in aqueous solution. Strain S. aureus A70, also isolated from commercial milk, produces a class II bacteriocin called aureocin A70 that inhibits a variety of Gram-positive bacteria, including also L. monocytogenes. Aureocin A70 is encoded by a mobilisable 8.0-kb plasmid, pRJ6, which has been found in other strains of S. aureus and coagulase-negative staphylococci. Aureocin A70 was the first bacteriocin described which is composed of four related small (30-31 amino acid residues) cationic peptides. Aureocin A70 is also expressed and excreted without post-translational modifications. Recently, a new bacteriocin produced by an S. simulans strain, simulancin 3299, has been identified by our group. This bacteriocin presents a high antagonistic activity against Streptococcus agalactiae involved in bovine mastitis. According to our knowledge, this was the first report on bacteriocin production by S. simulans.