This paper reports the solution conformation and calmodulin binding of a 43-residue peptide from the calmodulin-binding domain of Bordetella pertussis adenylate cyclase. The peptide (P225-267) was synthesized and 15N-labeled at specific amino acids. It binds calmodulin with an equilibrium dissociation constant of 25 nM. Assignment of the NMR spectrum of the free peptide and analysis of the NOE connectivities and secondary shifts of C alpha protons allowed us to identify a 10-amino acid fragment (Arg237 to Arg246) which is in rapid equilibrium between alpha-helical and irregular structures. Titration experiments showed that at substoichiometric molar ratios the two molecules are in intermediate exchange between free and bound conformations. Using 15N-edited methods we assigned a large part of resonances of the labeled residues in the bound peptide. Analysis of the chemical shift differences between free and bound states shows that the fragment Leu240-Ala257 is the most affected by the interaction. The proton spectra of the calmodulin, in the free and complexed states were extensively assigned using homonuclear experiments. Medium- and long-range NOE patterns are consistent with a largely conserved secondary and tertiary structure. The main changes in chemical shift of calmodulin resonances are grouped in six structural regions both in NH2- and COOH-terminal domains. Intermolecular NOE connectivities indicate that the NH2-terminal of the bound peptide fragment is engulfed in the COOH-terminal domain of calmodulin. The interaction geometry appears to be similar to those previously described for myosin light chain kinase or calmodulin kinase II fragments.