SummaryDuring an outbreak ofStaphylococcus aureusinfection in Brugmann Hospital, the investigation of isolated germ strains demonstrated the occurrence of a large variety of bacteriophage types. Most of them belong to group III.The pattern of bacteriophage types of all the strains isolated in the hospital is the same as that found in healthy carriers, in the nostrils of the nursing staff and in all strains isolated from staphylococcal lesions.Nevertheless, hospital infections are due, for the most part, to group III, in which most drug-resistant strains are to he found.Group I staphylococci were found in cutaneous lesions only. They kept a fairly high sensitivity to antibiotic.In this outbreak, the absence of a common source of infection implies a particularly great difficulty in eradicating the germs.