Summary: The production of bacitracin by Bacillus licheniformis closely paralleled growth in a synthetic medium without glucose. Glucose inhibited bacitracin production during the first hours of growth, whereas growth was not affected. Bacitracin was produced mainly during the later stages of growth. Formation of bacitracin was apparently not under catabolite repression control by glucose since the inhibitory effect of glucose upon the early bacitracin production was prevented by neutralizing the culture fluid with CaCO3. The inhibitory effect of glucose may be due to the low pH created by its metabolism. Addition of 0.5% glucose markedly increased the maximum titre of bacitracin. This stimulation could also be due to the effect of glucose metabolism upon the pH of the medium. The observation that peptide antibiotics are produced mainly after growth is not always true; in appropriate media they might also be produced during the phase of rapid growth.