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We investigated the relationship between Escherichia coli flagellar expression and the regulation of acetyl phosphate synthesis and degradation. Using cells either wild type for acetyl phosphate metabolism or defective for phosphotransacetylase or acetate kinase, or both, we measured flagellar expression and the intracellular concentration of acetyl phosphate relative to growth phase and temperature. Under the conditions tested, we found that elevated levels of acetyl phosphate corresponded to inhibition of flagellar synthesis. To extend these observations, we measured the intracellular concentration of acetyl‐CoA, the level of expression from the pta and ackA promoters, and the activities of phosphotransacetylase and acetate kinase derived from cell lysates. Relative to increasing culture density, acetyl‐CoA levels and expression from both the pta and ackA promoters decreased. Relative to Increasing …