To understand why proteins adopt particular three-dimensional structures, it is important to elucidate the hierarchy of interactions that stabilize the native state. Proteins in partly folded states can be used to dissect protein organizational hierarchies. A partly folded apomyoglobin intermediate has now been characterized structurally by trapping slowly exchanging peptide NH protons and analyzing them by two-dimensional 1H-NMR (nuclear magnetic resonance). Protons in the A, G, and H helix regions are protected from exchange, while protons in the B and E helix regions exchange freely. On the basis of these results and the three-dimensional structure of native myoglobin, a structural model is presented for the partly folded intermediate in which a compact subdomain retains structure while the remainder of the protein is essentially unfolded.