A soluble enzyme activity which catalyzes the synthesis of acyl-acyl carrier protein from acyl carrier proteins, a long chain fatty acid, and ATP has been demonstrated in E. coli. The reaction requires high concentrations of both Ca++ and Mg++ for activity, and cleaves ATP to AMP and PPi. The fatty acyl product has been identified as acyl-acyl carrier protein by its solubility, thioester linkage, molecular weight, charge, and biological activity. Several criteria indicate the enzyme is distinct from acyl-CoA synthetase. The fatty acid specificity of the enzyme suggests a role of acyl-acyl carrier protein synthetase in the incorporation of fatty acids into phospholipid.