Phycobilisomes (PBS) are supramolecular, multiprotein complexes which function as the light-harvesting antennae for Photosystem II (PS II) in the cyanobacteria, the chloroplasts of red algae, and the cyanelles of phylogenetically ambiguous flagellates such asCyanophora paradoxa (1). PBS occur as highly ordered arrays on the stromal surfaces of the thylakoids; each is believed to interact with two or more PS II reaction centers. PBS are predominantly composed of phycobiliproteins, a family of water-soluble proteins that carry linear tetrapyrrole chromophores (phycobilins; see 2). Each phycobiliprotein consists of two dissimilar subunits, denoted α and β, each of which carries 1–4 chromophores. The fundamental structural unit of the phycobiliproteins is the (αβ)3 trimer, a torroidal molecule about 11 nm indiameter and 3–3.5 nm in thickness with a central cavity about 3.5 nm in diameter (3, 4). Face-to-face stacking of two trimers produces (αβ)6 hexamers which are 11 × 6 nm. The assembly of larger substructures and PBS themselves requires the participation of a second class of proteins, the so-called “linker polypeptides,” which are absolutely required for the assembly of higher order structures (see below; also see 1, 2). The linker polypeptides probably fill the central cavity of the phycobiliprotein torroids and participate in the tail-to-tail joining of pairs of hexamers.