The acyl-carrier protein (ACP) of Escherichia coli is a protein of molecular weight 8847 with a 4'-phosphopanthetheine prosthetic group. ACP functions (via the SH of the prosthetic group) as a coenzyme in the synthesis of fatty acids and complex lipids. We report proton nuclear magnetic resonance (NMR) studies of the structure of ACP under various experimental conditions. The motion of the fatty acylchain of acyl-ACP has been investigated by 19F NMR studies of difluorotetradecanoyl-ACP. 31P NMR studies of the prosthetic group phosphorus of ACP and acyl-ACP are also reported. We make the following conclusions: (1) The structure of ACP is stabilized by surface charge, and (2) the fatty acid residue of acyl-ACP does not move freely and seems immobilized by an interaction with the protein moiety.