A strain of Bact. lactis aerogenes (Aerobacter aerogenes) was used in the study of adaptation to changes in the synthetic medium used, which contained potassium dihydrogen phosphate (brought to a pH of 7.12 with NaOH), MgSO4, glucose, (NH4)2SO4, and distilled water. Growth curves (log count/ml. against time) showed a rapid and reversible adaptation to a change in the source of carbon from glucose to glycerol. The same occurred when a change in the source of N from (NH4)2SO4 to glycine was made. In both cases, adaptation was practically complete after 3-4 transfers in the 2d medium. Expts. with washed and unwashed cells indicated that the changes were really due to adaptation and not to the transfer of medium with the inoculum. Of the hypotheses advanced to explain the mode of adaptation, the authors considered that a change in the balance of enzymes controlling growth in the lag phase to be the most satisfactory one.