The dense and standard cultures of a thermophilic cyanobacterium, Chroococcidiopsis sp. strain TS-821 have been studied (Ishida et al., J. Ferment. Bioeng., 83, 571-576). When freeze-dried cells from the dense culture of strain TS-821 were suspended in BED buffer, the supernatant following centrifugation was clear blue. A blue color protein was purified from the supernatant using Superose 6 and Mono Q columns. The N-terminal sequence of the two subunits and absorption and fluorescence spectra indicated that the purified protein was C-phycocyanin. The molecular weight measured by gel filtration is 600 kDa, which is quite a high molecular weight. The results of Western blot analysis and the ratio of A(614)/A(280) indicated that C-phycocyanin was produced in larger amounts in the dense culture compared to in the standard culture. Because a higher amount of phycocyanin was produced in the dense culture, a higher amount of phycocyanin seemed to be extracted into BED buffer.