Among 150 strains, including marine cyanobacteria isolated from coastal areas of japan and a freshwater cyanobacterium from the IAM collection, Spirulina platensis IAM M-135, the marine cyanobacterium Synechococcus sp. NKBG 042902 contained the highest amount of phycocyanin (102mg/g dry cell weight). We have proposed that the cyanobacterium could be an alternative producer for phycocyanin. The effects of light intensity and light quality on the phycocyanin content in cells of Synechococcus sp. NKBG 042902 were investigated. When the cyanobacterium was cultured under illumination of 25 mu molm(-2)s(-1) using a cool-white fluorescent lamp, the phycocyanin content was highest, and the phycocyanin and biomass productivities were 21mgl(-1)day(-1) and 100mgl(-1)day(-1) respectively. Red light was essential for phycocyanin production by this cyanobacterium. Phycocyanin and biomass production were carried out by the cyanobacterium cultures grown under only red light (peak wavelength at 660nm) supplied from light-emitting diodes (LED). Maximum phycocyanin and biomass productivities were 24 mgl(-1) day(-1) and 130 mgl(-1)day(-1) when the light intensity of the LED was 55 mu molm(-2)s(-1).