Phytochrome was isolated from etiolated rice seedlings (Oryza sativa L.) by slight modification of the procedure for oat phytochrome. Spectral data of rice phytochrome are comparable with those of oat and rye phytochrome. Controlled proteolysis with endoproteinases Lys-C and Glu-C yielded defined fragments some of which were different for Pr and Pfr. The fragments were identified by comparison with the corresponding fragments of oat phytochrome and by comparison of the amino acid sequences of rice and oat phytochrome. Regions of the peptide chain which are differently exposed in Pr and Pfr were identified. A highly conserved sequence around residues 740-750 is discussed as candidate for an "active center" of signal transduction.