Pseudanabaena 7409 is a chromatically cyanobacterium which photocontrols the synthesis of both phycoerythrin and phycocyanin [Tandeau de Marsac (1977) J. Bacteriol. 130, 82--91]. Phycobilisomes, isolated from cells grown in either green or red light, have been dissociated and the component biliproteins purified and characterized. Phycobilisomes isolated from cells grown in green light were composed of allophycocyanin B, allophycocyanin, two phycocyanin subunits (one alpha-type and one beta-type subunit), phycoerythrin and eight uncolored polypeptides. When dissociated phycobilisomes were chromatographed on DEAE-cellulose at pH 5.5, most of the phycocyanin was recovered as part of a large (17.3 S) multiprotein complex with phycoerythrin (molar ratio 1 : 1). This complex also contained five of the uncolored polypeptides found in intact phycobilisomes isolated from cells grown in green light. Phycobilisomes isolated from cells grown in red light were composed of allophycocyanin B, allophycocyanin, four phycocyanin subunits (two alpha-type and two beta-type subunits), and six uncolored polypeptides. When these phycobilisomes were dissociated, the phycocyanin was recovered as a large (21.0 S) multiprotein complex which was composed of the four phycocyanin subunits types and four uncolored polypeptides. This complex was morphologically identical to the rod-like stacks of discs about 6 x 12 nm which form the peripheral rods of intact phycobilisomes. Each of the four phycocyanin subunits found in the complex isolated from the phycobilisomes of cells grown in red light was purified to homogeneity and characterized. Amino acid compositions of the four subunits indicated that each subunit was a unique gene product. Two of the subunits of the complex were apparently identical to those of the phycocyanin purified from phycobilisomes isolated from cells grown in green light. These studies suggest that one pair of phycocyanin subunits was synthesized constitutively (i.e. irrespective of the light wavelength to which the cells were exposed during growth) while the synthesis of the second pair of phycocyanin subunits was specifically induced during growth in red light.