All available isolates of coagulase-negative staphylococci from patients admitted to a unit for cardiac surgery during a 14-month period were examined. During this time 382 operations, in 220 of which a prosthetic valve was implanted, were performed. 65 patients suffered 76 episodes of postoperative infection. All coagulase-negative staphylococci isolated were classified by biochemical typing, bacteriophage typing and testing of anitbiotic susceptibility. The results of this investigation were reported and discussed especially from an epidemiological point of view.