In order to fix CO2 generated by microbial fermentation, two astaxanthin over-producing microorganisms, the green alga Haematococcus pluvialis and the red yeast Phaffia rhodozyma, were mix-cultivated in the same media. CO2 from P. rhodozyma fermentation was fixed by H. pluvialis simultaneously in the process of photosynthesis, while O-2 produced by H. pluvialis in photosynthesis stimulated astaxanthin formation in P. rhodozyma. As a result, both concentrations of biomass and astaxanthin increased significantly compared to the pure cultures of the two species. The maximum concentrations of biomass and astaxanthin, 5.70 g/l and 12.95 mg/l, were obtained by the mixed culture. This culture strategy provides a novel way for improving the yield of higher valued bio-product with in situ CO2 fixation. (C) 2004 Elsevier B.V. All rights reserved.