Infrared spectra for the carbon monoxide complex with myoglobin isolated as the oxygenyl species from bovine heart muscle were carefully examined in the C--O stretch region as either the pH or the temperature was varied. Deconvolutions of these spectra into bands of Gaussian shape suggest the presence of four bands near 1938(I), 1944(II), 1954(III), and 1965(IV) cm-1 with halfband widths of about 18, 9, 9, and 10 cm-1, respectively. The relative intensities of the four bands varied with changes in pH or temperature. 13C NMR spectra and other evidence indicate that the four C--O stretch bands arise from four discrete rapidly interconverting conformers: CI, CII, CIII, and CIV. Under conditions of physiological pH and temperature, the relative stabilities are CI approximately CII much greater than CIII approximately CIV. The delta H and delta S values for conformer interconversions are estimated to range from -8 to 34 kJ/mol and -27 to 87 J.mol-1 K-1, respectively; therefore the structures of the conformers may be expected to vary significantly. These findings provide evidence for a highly flexible, dynamic structure at the ligand-binding site of bovine myoglobin, even when ligands are bound.