Abstract 1. 1. The optical rotatory dispersion (ORD) and circular dichroism (CD) of phytochrome in the P R and P FR forms were determined from 200 to 500 nm. Both the ORD and the CD spectra showed dramatic differences between the P R and P FR in this wavelength range. 2. 2. The CD spectra below 240 nm correspond closely with that assigned to the β-chain configuration. If, however, the protein is considered to be a mixture of α-helix and random coil, the ellipticity at 220 nm would correspond to an α-helix content of 14%.