The presence of sequences related to the agr of Staphylococcus aureus was demonstrated in Staphylococcus epidermidis by agr -specific PCR, and Southern blot. The agr -like locus of S. epidermidis A086 was cloned and sequenced. An overall homology of 68% was found between the agr locus from S. epidermidis and S. aureus . The agr locus from S. epidermidis was organized similar to those from S. aureus and S. lugdunensis . The putative RNAII molecule contains four open reading frames, agrA , B , C and D . AgrA was a response regulator. AgrB showed homology with transducer and translocase molecules. AgrC is expected to act as a histidine protein kinase in which a leucine zipper is present. AgrD is presumably processed into an autoinducer peptide. The putative RNAIII molecule contained an open reading frame encoding a putative 26 amino acid (aa) polypeptide, which differed in 3 aa from the RNAIII encoded δ-toxin of S. aureus . Kinetic studies showed that the production of this RNAIII was elevated during the post-exponential phase. δ-Toxin activity was demonstrated for 21 of 23 tested S. epidermidis strains. Kinetic studies of the production of δ-toxin showed that the toxin was produced during the post-exponential phase. Sequencing of S. epidermidis A097, which showed a delayed agr -response, revealed a truncated AgrC lacking the histidine kinase domain. These data indicate that an agr -like locus is active in S. epidermidis during the post-exponential phase.