Phase-sensitive two-dimensional nuclear magnetic resonance (n.m.r.) experiments have been used to obtain extensive proton resonance assignments for the carbon monoxide complex of sperm whale myoglobin. Multiple quantum experiments were particularly important in the assignment procedure. The assignments are the most complete yet reported for a protein of such high molecular weight (approximately 18,000) and make possible new and comprehensive studies of the structure and dynamics of carbonmonoxymyoglobin in solution. Assignments for seven of the histidine residues are reported, including the critical proximal and distal histidines. Most of these are at variance with the assignments already in the literature. The present n.m.r. data indicate that histidines 24 (B5) and 119 (GH1) are hydrogen bonded to each other and, in contrast to neutron diffraction data, show that His24 does not protonate at pH greater than 5. The aromatic rings of all the phenylalanine and tyrosine residues undergo rapid flips about the ring axis. The side-chains of Leu89 (F4) and Phe138 (H15), which border a large hydrophobic cavity, are particularly mobile.