A study was made of the pigment composition and ultrastructure of a unicellular cyanophyte living in symbiosis with colonial didemnid ascidians and encrusting sponges collected from the southern end of the Great Barrier Reef, Australia, in 1981-1984. The ascidians were Trididemnum tegulum Kott and T. clinides. Kott; the sponges were Prianos aff. melanos de Laubenfels, Spirastrella aff. decumbens Ridley and an unidentified brown fleshy sponge (BFS). This cyanophyte seems to be identical with Synechocystis trididemni Lafargue et Duclaux. A phycoerythrin containing both phycourobilin and phycoerythrobilin chromophores was shown to be present; the urobilin was carried on α and β subunits, no γ subunit was found. A second phycoerythrin possessing only erythrobilin chromophores was also present. In thin-sections the cells showed no central DNA-containing nucleoid, and an unusual thylakoid arrangement with some thylakoids having greatly expanded lumens forming pseudo-vacuoles in the centre of the cell. Freeze-fracture showed 11 to 12 nm particles on both PF (protoplasmic face) and EF (exoplasmic face) faces of thylakoids. In many ways, the ultrastructure resembled that of the chlorophyll-b containing prokaryote Prochloron spp.