The structural formula of the novel antibiotic ionophore CP-54, 883 is deduced by systematic reduction from its ¹³C and ¹H NMR spectra. The molecule consists of a polyether ring network and side chain terminated by an aromatic ring containing a phenoxy and two chlorine substituents. Based partly on an assumed analogy to corresponding regions of the similar structure nigericin-A₁, the configurations about the sixteen asymmetric carbons are also derived. A belated crystal structure determination confirms, with the exception of one configuration assumed from nigericin-A₁, the conclusions drawn, and shows that the anionic charge is in the phenoxy group, rather than the carboxylic acid function.