The cyanobacterium Spirulinia platensis is an attractive alternative source of the pigment chlorophyll, which is used as a natural color in food, cosmetic, and pharmaceutical products. In this work, the influence of the light intensity and urea supplementation as a nitrogen source using fed-batch cultivation for S. platensis growth and chlorophyll content was examined. Cultivations were carried out in 51 open tanks, at 30 +/- 1 degreesC. Response surface methodology was utilized for analysis of the results, and models were obtained for biomass productivity, nitrogen-cell conversion factor and chlorophyll productivity. The best cellular growth was observed with 500 mg/l of urea at a light intensity of 5600 1x, whereas the highest concentration of chlorophyll in the biomass was observed with 500 mg/l of urea at a light intensity of 1400 1x. Overall, the best chlorophyll productivity was observed with 500 mg/l of urea at a light intensity of 3500 1x, providing the optimal balance between the cellular growth and the biomass chlorophyll content. (C) 2003 Elsevier Ltd. All rights reserved.