During the solution structure determination of the Escherichia coli quorum-sensing protein SdiA in the presence of N-octanoyl-l-homoserine lactone (HSL), NMR signals were detected in (13)C-filter-(13)C-filter spectra for the bound HSL molecule. An additional set of coupled signals, independent of those of HSL, were also detected, indicating the presence of another unlabeled molecule, also bound to the labeled SdiA. Analysis of the NMR spectrum of this ligand and of the mass spectrum of the dissociated components indicates that the ligand is most likely xylose. Further analysis of xylose-bound SdiA defines a site close to the C terminus, remote from the HSL binding site. These observations provide an example of the sensitivity of high-resolution NMR experiments and their ability to detect, identify, and map the adventitious binding of a small organic molecule to a protein.