We describe the isolation and interaction with calmodulin (CaM) of two 10-amino-acid peptides (termed peptides 1 and 2; AWDTVRISFG and AWPSLQAIRG respectively) derived from a phage random peptide display library. Both peptides are shorter than previously described CaM-binding peptides and lack certain features found in the sequences of CaM-binding domains present in CaM-activated enzymes. However, 1H NMR spectroscopy and fluorimetry indicate that both peptides interact with CaM in the presence of Ca2+. The two peptides differentially inhibited CaM-dependent kinases I and II (CaM kinases I and II) but did not affect CaM-dependent phosphodiesterase. Peptide 1 inhibited CaM kinase I but not CaM kinase II, whereas peptide 2 inhibited CaM kinase II, but only partially inhibited CaM kinase I at a more than 10-fold higher concentration. Peptide 1 also inhibited a plant calcium-dependent protein kinase, whereas peptide 2 did not. The ability of peptides 1 and 2 to differentially inhibit CaM-dependent kinases and CaM-dependent phosphodiesterase suggests that they may bind to distinct regions of CaM that are specifically responsible for activation of different CaM-dependent enzymes.