Infections with the leading nosocomial pathogen Staphylococcus epidermidis are characterized by biofilm development on indwelling medical devices. We demonstrate that the quorum-sensing regulator agr affects the biofilm development of S. epidermidis in an unexpected fashion and is likely involved in promoting biofilm detachment. An isogenic agr mutant showed increased biofilm development and colonization in a rabbit model. In addition, nonfunctional agr occurred more frequently among strains isolated from infections of joint prostheses. Lack of functionality was based on mutations, including insertion of an IS256 element. Relative to other bacterial pathogens, quorum sensing in S. epidermidis thus has a different role during biofilm development and biofilm-associated infection. Our results indicate that disabling agr likely enhances the success of S. epidermidis during infection of indwelling medical devices. The permanent elimination of quorumsensing regulation used by S. epidermidis represents a surprising and unusual means to adapt to a certain environment and type of infection.