The pyrenoid is a prominent proteinaceous structure found in the stroma of the chloroplast in unicellular and multicellular algae, and contains the majority of the cellular ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco), a key enzyme in the Calvin-Benson cycle of photosynthetic CO2, assimilation. Pyrenoids from the marine unicellular algae Dunaliella tertiolecta were isolated after the treatment of the chloroplast with mercurium chloride. Transmission and scanning electron microscopy showed that tubule-like structures enter into the intrapyrenoidal space. SDS-PACE of the pyrenoids showed that most of the components consisted of the large (55 kDa) and small subunits (15 kDa) of Rubisco, and several minor polypeptides (52 and 18 kDa). The possible role of the pyrenoid in CO2 assimilation is discussed.