Using two- and three-dimensional NMR techniques, 1H and main-chain 15N resonances of the N-terminal half domain of yeast calmodulin (YCM0-N) in the presence of Mg2+ and Ca2+ (Mg(2+)-and Ca(2+)-forms) were assigned. The secondary structures of YCM0-N in both forms were determined. The NOESY and 15N-edited NOESY spectra of YCM0-N in each form indicate that there is a hydrophobic core and that two Ca(2+)-binding loops are connected by a short antiparallel beta-sheet. There are four helices (A, B, C, and D named from the N-terminus) for YCM0-N in the Mg(2+)-form. The B-helix is, however, not formed in the Ca(2+)-form. The Ca(2+)-binding of YCM0-N was monitored by (1H,15N)-HSQC at various Ca2+ concentrations. The observed spectral changes as a function of Ca(2+)-concentration can not readily be grouped into a small number of classes; each residue shows individual spectral change. There is no apparent relationship between the spectral change and the type or location of the amino acid concerned.