A thermophilic cyanobacterium, Chroococcidiopsis sp. strain TS-821, was cultivated under illumination at various intensities of and at various concentrations of medium components in a 1-l photobioreactor. The specific growth rate and the dry cell weight at the end of cultivation were determined. On the basis of the results obtained, cultivation under the optimum conditions was carried out for achieving a dense culture. Cultivation was also done under conditions of CO2 supply in two different modes : (i) 10% CO2 throughout, and (ii) 10% at the beginning and then 20% at the termination of linear growth. The dry cell yield per liter at the end of the culture was 70% higher than that in the case of standard culture. The total sugar content of the cells reached 30% of the dry cell weight at the end of the dense culture, which is higher than that of other cyanobacterium, Synechococcus elongatus PCC 6301. The supernatant of a buffer suspension of cells which had been collected at the end of the dense culture and freeze-dried was clear blue. The absorption spectrum of this supernatant was measured and compared with those of the cell extract and of a phycobiliprotein extracted from cell of this strain. The wavelengths of two absorption maxima from the supernatant coincided with those from the phycobiliprotein. The results show that the phycobiliprotein can easily be excreted from the cells. The results of polyacrylamide gel electrophoresis and spectrophotometrical analyses suggest that the excreted phycobiliprotein is a phycocyanin. Furthermore, when total protein, lipid, and sugar contents were measured without removal of the culture medium, the total sugar content was about 50% of the cell component. This indicates that the sugar was excreted into the medium.