BACKGROUND: Recent studies indicate that microalgal cultivation using organic carbon sources has the potential to provide high yields. Haematococcus pluvialis and Chlorella zofingiensis, two important carotenoid producers, were selected for co-culture cultivations to utilize the unique advantages of both organisms. A co-culture production process was investigated in terms of the effects of organic carbon source, co-cultivation method, and light intensity on carotenoid production. RESULTS: The addition of 5 g L-1 glucose resulted in a growth rate of 0.60 day(-1) for H. pluvialis and 0.59 day(-1) for C. zofingiensis, which were higher than those for other carbon sources tested and the control group. Incremental increase of light intensity instead of direct increase to 170 mu E m(-2)s(-) prevented cell loss in both cultures. Co-cultivation based on cell numbers (60% H. pluvialis and 40% C. zofingiensis) prevented population domination of one microalgae over the other. The biomass production rate of the co-culture was higher (0.61 g L-1 day(-1)) in glucose-enriched medium. The total carotenoid content of the co-culture in the control culture was higher (0.83 mg total carotenoids g(-1) cell) than that obtained in glucose-enriched medium (0.54 mg total carotenoids g(-1) cell) but not as high as the amounts reached in mono-cultures. CONCLUSION: Total carotenoid content of the mono-cultures gave higher yields in standard bold basal medium (BBM). Preliminary high performance liquid chromatography (HPLC) studies indicated a variation in the amounts of astaxanthin isomers produced. Further studies are in progress to determine the effects of carbon-enriched media and co-cultivation on the type of isomers and caretenoids produced. (C) 2010 Society of Chemical Industry