The structure and dynamics of two partially folded states of apomyoglobin have been characterized at equilibrium using multi-dimensional NMR spectroscopy. Residue-specific measurements of chemical shift and internal dynamics in these states and in the native apoprotein and holoprotein indicate progressive accumulation of secondary structure and increasing restriction of backbone dynamics as the chain collapses to form increasingly compact states. Under weakly folding conditions, the polypeptide fluctuates between unfolded states and local elements of structure that become extended and stabilized as the chain becomes more compact. These results provide a detailed model for molecular events that are likely to occur during folding of myoglobin.