Representatives of phycobiliprotein-containing algae such as Anabaena cylindrica, Anacystis ni- dulans, Gloeotrichia echinulata (Cyanophyceae), Chroomonas spec., Hemiselmis rufescens, Rhodomonas spec. (Cryptophyceae), Porphyridium cruentum, Rhodella violacea (Rhodophyceae) along with the unicellular Cyanidium caldarium (unspecified systematic status) have been investigated for their typical photoassimilatory accumulation products. While the red algal species synthesize a rather specific heteroside, 2-O-ᴅ-glycerol-α-ᴅ-galactopyranoside (= floridoside), not encountered in the other species analyzed, blue-green algae accumulate fructose, glucose, and sucrose, while the cryptomonads accumulate only glucose and. to a lesser extent, fructose. Cyanidium synthesizes neither disaccharides, nor a heteroside, but shows rapid 14 C-labelling of fructose and glucose. These results are compared with further biochemical and structural findings and are discussed with emphasis on chemotaxonomic implications. Cyanidium caldarium is proposed as an endocyanome consisting of a single endocyanella providing the functions of a chromatophore in a colourless (apoplastidal algal?) host cell.