It has been suggested that, in rod-shaped bacteria, two sites for peptidoglycan assembly exist: one which is responsible for septum formation and the other, for lateral wall extension. The balance between the activities of these two sites enables bacteria to conserve their own morphology during cell growth. The effect of specifically inhibiting septum formation by different means (antibiotics and/or mutations), upon cell surface extension and macromolecular synthesis in rod-shaped and coccoid bacteria of various species, was studied. Inhibition of either cell wall expansion or macromolecular synthesis did not occur when septum formation was impaired in both rod-shaped bacteria and cocci possessing the two sites for peptidoglycan assembly, whereas a rapid and complete block of such synthesis was caused by inhibiting both sites in rod-shaped bacteria, or septum formation in cocci which possess only this site. These data indicate that bacteria possess a control mechanism that prevents macromolecular synthesis when envelope extension is inhibited.