Optical rotatory dispersion measurements of solutions of sperm whale ferrimyoglobin and horse ferrihemoglobin have been performed at wavelengths between 225 m μ and 730 m μ. Analysis of these results indicates that both proteins in dilute aqueous solution depart considerably from the behavior observed with most native globular proteins. This departure is attributed to unusually high helix contents in solution. The helix content has been estimated to be around 80% for both proteins from b 0 values using the Moffitt equation in the wavelength range 236 to 334 m/ μ,. The helix content has also been estimated using the intensity of the optical rotation trough at 233 m μ ( Simmons, Cohen, Szent-Györgyi, Wetlaufer & Blout, 1961). This estimate yielded values of about 75% helix content for both ferrimyoglobin and ferrihemoglobin. In addition, for both proteins, two positive Cotton effects have been observed in the visible. Their inflection points correspond to prominent absorption bands. The relationship of the structure of the molecules in dilute solution to that in the wet crystal is discussed.