The thermal expansion of a protein, metmyoglobin, was investigated by analysis of the refined X-ray crystal structures at 80 and 255-300 K. On heating from 80 to 300 K, the volume occupied by myoglobin increases by approximately 3%. The linear thermal expansion coefficient is estimated to be 115 X 10(-6) K-1. This value is more than twice as large as that of liquid water but less than that of benzene. As the temperature is raised, the internal volume change does not come from the large, atom-sized internal cavities in the structure but from an increase in the small, subatomic free volumes between atoms. The largest expansion occurs in the region of the CD and GH corners; both these regions move away from the center of the protein. The remainder of the expansion results from the lengthening of contacts between segments of secondary structure.