The carotenoid compositions of Phormidium persicinum, P. luridurn, P. faveolarum and Anabaena flos-aquae have been studied, both quantitatively and qualitatively. β-Carotene is the major carotenoid in all species. The xanthophylls comprise zeaxanthin, echinenone, canthaxanthin and the furanoid mutatochrome. Phormldiumpersicinum lacks glycosidic carotenoids. Myxoxanthophyll (myxol-2'-rhamnoside) and a 4-ketomyxol-2'-methylpentoside (tentatively 4-keto-myxoxanthophyll) are present in the other species. These distribution patterns are compared with those observed in other blue-green algae and some correlations with taxonomy are apparent.