In photochromic pigments, immobilized on Sepharose beads to prevent reorientation of the molecules, linear absorption dichroism can be induced with plane‐polarized light. This phenomenon was used to study the relation between the carrier protein and the chromophore in the photochromic cyanobacterial pigment, phycochrome b, from Tolypothrix distorta var. symplocoides Hansgirg, strain UTEX 424 (formerly IUCC 424). It was found that the transition moment of the chromophore has the same direction with respect to the protein in the long‐wavelength and the short‐wavelength forms of phycochrome b. This contrasts to earlier results for phytochrome in higher plants, in which the transition moment was found to rotate 32° (or 180°—32°) when the long‐wavelength form was converted to the short‐wavelength form or vice versa.