The aromatic residues of fd coat protein in sodium dodecyl sulfate micelles are characterized by 1H and 13C NMR. Resonances from both types of nuclei show structure-induced chemical shift dispersion and line widths indicative of a folded native structure for the protein. The two tyrosines were found to have pKas of 12.3 and 12.5 by 1H NMR and spectrophotometric titrations. 13C relaxation measurements show that two of the three Phe rings have significant internal mobility, the two Tyr rings have moderate internal mobility, and the Trp side chain is completely immobilized. Qualitative comparisons are made between the intact virus and the isolated coat protein.