Enzymes involved in the synthesis of teichoic acid and its linkage to the wall in Bacillus subtilis W23 were measured in chemostat cultures growing at equilibrium at a dilution rate of 0.2 h-1 in different concentrations of inorganic phosphate. All the enzymes, except teichoic acid glucosyl transferase, which was insensitive to changes in phosphate concentration, were almost undetectable at 0.5 mM-phosphate. At higher phosphate concentrations the changes in activity of the enzymes of linkage unit synthesis were sufficient to account for the changes in the rate of incorporation of teichoic acid into the wall in vivo. Between 3.5 and 4.5 mM-phosphate the amount of teichoic acid synthesized in vivo increased, but no increase in the ability of toluenized bacteria to synthesize teichoic acid could be detected. Allosteric regulation might therefore be important at high phosphate concentrations. Bacteria maintained a constant ATP content and a constant adenylate energy charge during chemostat growth at all phosphate concentrations.