Prokaryotic green algae of the division Prochlo rophyta (Lewin 1976) have been reported in ob ligate association with at least 15 species in five genera of the family Didemnidae (Kott 1982) and non-obligate association with at least 21 species from a range of ascidian families (Kott et al 1984). Prokaryotic algal cells of the genus Prochloron have chlorophyll a and b but lack phycobilins. Lewin and Cheng (1975) found pink unicellular and filamentous cyanophytes (Cyanophyta) on the surface of Didemnum spp. in the Gulf of California. Cyanophytes of the genus Synecho cyst is have been reported in association with two species of ascidians in the Atlantic: Trididem num solidum Van Name (1902) (see Olson 1980; Symbesma et a11981) and Trididemnum cyano phorum Lafargue & Duclaux (1979). This is the first report of red cyanophytes with R-phycoerythrins in both obligate and non-ob ligate associations with ascidians from the Great Barrier Reef. All seven species were collected from cryptic habitats just below low water mark at Heron Island, Great Barrier Reef. Two of the species, Didemnum digestum Sluiter (1909) and Tridi demnum cerebriforme Hartmeyer (1913), had the algal cells on the surface. Although the associa tion with D. digestum is non-obligate, colonies were very rarely found without at least some part of the surface being covered with the algal cells. Kott et al (1984) referred to two colonies of T. cerebriforme in which only half the surface was covered with the cyanophyte while the other half was covered with Prochloron. Other colonies were exclusively covered with either the red cyano phyte or with Prochloron. In Trididemnum sp. (tegulum Kott, in press), T. clinides Kott (1977), T. miniatum Kott (1977), Didemnum viride (Herdman 1906) and D. aff sphaericum Tokioka (1967 ), the red cyanophytes are embedded in the test of the ascidian. The red pigment in fresh material of T. miniatum referred to by Kott (1980) is the red cyanophyte. It is present in the larvae, where it is trunk encased with the Prochloron cells, as well as in the adult colonies and is possibly transferred to the next generation in this way. In T. clinides and T. miniatum the red cyanophyte is mixed with the green Pro chloron cells. The cyanophytes from Trididemnum sp., T. clinides, D. viride, and D. aff sphaericum are all unicellular and can be assigned to the genus Sy nechocystis (G. Cox, pers. comm.). The red cy anophytes from D. digestum and T. cerebri forme, both in non-obligate association, appear from the ultrastructure to be different species. The alga in T. miniatum is a colonial filamentous cyanophyte. The cyanophytes were removed from the sur face of the D. digestum and T. cerebriforme col onies by brushing under a stream of filtered sea water. The cells were then centrifuged and ex tracted into 0.2 M phosphate buffer (pH 7.0) with freezing and thawing or were extracted with ace tone for thin layer chromatography. In Trididemnum sp., T. clinides, T. minia tum, D. viride, and D. aff sphaericum the algae are embedded in the test and not readily re moved. As a result whole colonies were extracted by crushing colonies in a glass homogenizer with 0.2 M phosphate buff er (pH 7.0), freezing and thawing. The residue was then extracted with acetone. All extractions were performed with buffered solutions because of the acid produced by these species when the bladder cells are dam aged (Hawkins et al 1984). Absorption spectra of the aqueous extracts of the phycobilins were measured immediately on a Varian DMS 90 spectrophotometer. The so lutions were frozen and stored under nitrogen until circular dichroism (CD) spectra were run