Ten strains of naturally penicillin-resistantStaphylococcus aureus(obtained from patients), two in vitro derived resistant strains, and two sensitive strains, were grown at 37 C in Antibiotic Assay broth, and viable cell determinations were made at intervals. From these data, growth curves were plotted for each of the strains. The curves for the naturally penicillin-resistant and the sensitive strains are very similar. Little, if any, lag in growth of these strains occurred on transfer from maximum stationary-phase cultures to fresh medium. They grew at approximately the same rate during the logarithmic growth phase, which lasted for 3 to 4 hours; during the maximum stationary phase, about the same number of cells was present per milliliter in cultures of each of these strains. In contrast, the in vitro derived resistant strains underwent a lag of 2 to 6 hours on transfer to fresh medium and grew at a slower rate during the logarithmic growth phase. However, during the maximum stationary phase, which occurred after an incubation period of 24 to 32 hours, the cell titers were approximately the same as those of the naturally resistant and the sensitive strains. When grown in competition with either of the sensitive strains in Antibiotic Assay broth in the absence of penicillin, one of the naturally resistant strains persisted for 14 successive subcultures without any apparent change in ability to tolerate the antibiotic.