We present here the complete primary structure of R-phycocyanin-I a and p subunits from the red alga Porphyridium cruentum. The a chain is composed of 162 amino acid residues (18049Da, calculated from sequence, including chromophore) and carries a phycocyanobilin pigment covalently linked to Cys84. The p chain contains 172 amino acids (19344Da, calculated from sequence, including chromophores) and carries a phycocyanobilin pigment covalently linked at Cys82 and a phycoerythrobilin pigment at Cys153. A y-N-methyl asparagine residue was also characterised at position ,872 similar to other phycobiliprotein , 6 subunits. R-phycocyanin-I from Porphyridium cruentum shares high sequence identity with C-phycocyanins (69 -83%), R-phycocyanins (66-70%) and in a less extent with phycoerythrocyanins (57-65%) from various sources.The presented phylogenetic trees are based on a comparison of all phycobiliprotein amino acid sequences known so far and confirm the clear affiliation of the R-phycocyanins in the phycocyanin family. In spite of their particular phycobilin pattern, they do not represent intermediate forms between the phycocyanin and the phycoerythrin family. Phycoerythrocyanin, a phycocyanin-related phycobiliprotein adapted to green light harvesting, is also shown to belong to the phycocyanin family. However, the phycoerythrocyanins diverge from phycocyanins in their different function and it is suggested that they should be assigned to a separate group within the phycocyanin family.Phycobilisomes represent the main accessory light-harvesting antenna of cyanobacteria and red algae (Cohen-Bazire and Bryant, 1982;Zuber, 1983Glazer, 1984Glazer, , 1985Gantt, 1988;Bryant, 1991; Sidler, 1994). Their major components, the phycobiliproteins, are constituted of a and p subunits (16-18 kDa and 18-20 kDa respectively) carrying one, two or three open-chain tetrapyrroles (bilins) per subunit as prosthetic groups. Both subunits are aggregated in vivo to heterotrimers (an3 or hexamers (anb. Electron microscopy and crystal structure analysis show that the trimer complex is the basic building block of the phycobilisome (Schirmer et al., 1985). The ap monomer aggregates in a disc-shaped complex with a diameter of 11 nm and a length of 3 nm. A hexamer is built up of two trimers stacked face-to-face (Schirmer et al., 1986). The formation of higher aggregatesCorrespondence to H.