All but four of twenty-five strains of Staphylococcus aureus exposed to concentrations of 60 μg. benzylpenicillin/ml. (or 100–500 μg. methicillin/ml. for penicillinase-producing strains) on nutrient agar media containing 3.5 % (w/v) sodium chloride and 10 % (v/v) horse serum, gave rise to typical L colonies. The L forms were subcultured on nutrient agar and in broth containing 3.5 % sodium chloride and were found to be completely resistant to the penicillins, cycloserine, ristocetin, vancomycin and cephalosporin, but to be sensitive to other antibiotics, often in slightly lower concentrations than the parent cocci. The L forms produced coagulase and, in two of three strains tested, were lysogenic although resistant to phage lysis. In studies of the transformation of staphylococci to L forms, it was noted that a variable but often very large proportion of the early L microcolonies failed to develop into typical colonies.