The ubiquitous Ca2+-binding regulatory protein calmodulin (CaM) binds and activates a wide range of regulatory enzymes. The binding is usually dependent on the binding of Ca2+ to CaM; however, some target proteins interact with CaM in a calcium-independent manner. In this work, we have studied the interactions between CaM and a 20-residue synthetic peptide encompassing the major calmodulin-binding domain of cyclic nucleotide phosphodiesterase (PDE1A2). The binding was studied in the absence and presence of Ca2+ by far-UV and near-UV circular dichroism, fluorescence, and infrared spectroscopy. In addition, two-dimensional heteronuclear NMR studies with 13C-methyl-Met-CaM and uniformly 15N-labeled CaM were performed. Competition assays with smooth muscle myosin light chain kinase revealed a Kd of 224 nM for peptide binding to Ca2+-CaM, while binding of the peptide to apo-CaM is weaker. The peptide binds with an alpha-helical structure to both lobes of Ca2+-saturated CaM, and the single Trp residue is firmly anchored into the C-terminal lobe of CaM. In contrast, the Trp residue plays a minor role in the binding to the apo-protein. Moreover, when bound to apo-CaM, the PDE peptide is only partially helical, and it interacts solely with the C-terminal lobe of CaM. These results show that the Ca2+-induced activation of PDE involves a significant change in the structure and positioning of the CaM-bound PDE peptide domain.