1. Klebsiella aerogenes contains two different acyl carrier proteins, one specific for citrate lyase, the other for fatty acid synthetase. 2. The acyl carrier protein of fatty acid synthetase from K. aerogenes was isolated and compared with the corresponding protein from Escherichia coli and with the acyl carrier protein of citrate lyase from K. aerogenes. 3. As judged from prosthetic group compositions as well as amino acid and fingerprint analyses, the acyl carrier proteins of the two fatty acid synthetases are nearly identical but different from that of citrate lyase from K. aerogenes. 4. Therefore, the different prosthetic groups alone cannot be responsible for the different specificities of the acyl carrier proteins of fatty acid synthetase and citrate lyase in K. aerogenes. 5. The prosthetic group of citrate lyase, phosphoribosyl dephospho-CoA, apparently represents no incidental, phosphopantetheine-replacing aberration. The requirement of citrate lyase for the CoA-like prosthetic group may arise from the substrate requirement of both subunit enzymes of the enzyme complex.