The freshwater microalga Haematococcus pluvialis is one of the best microbial sources of the carotenoid astaxanthin. but this microalga shows low growth rates and low final cell densities when cultured with traditional media. A single-variable optimization strategy was applied to 18 components of the culture media in order to maximize the productivity of vegetative cells of H. pluvialis in semicontinuous culture. The steady-state cell density obtained with the optimized culture medium at a daily volume exchange of 20% was 3.77 . 10(5) cells ml(-1), three times higher than the cell density obtained with Bold basal medium and with the initial formulation. The formulation of the optimal Haematococcus medium (OHM) is (in g l(-1)) KNO3 0.41, Na2HPO4 0.03, MgSO4 . 7H(2)O 0.246, CaCl2 . 2H(2)O 0.11, (in mg l(-1)) Fe(III)citrate . H2O 2.62, CoCl2 . 6H(2)O 0.011, CuSO4 . 5H(2)O 0.012, Cr2O3 0.075, MnCl2 . 4H(2)O 0.98, Na2MoO4 . 2H(2)O 0.12, SeO2 0.005 and (in yg l(-1)) biotin 25, thiamine 17.5 and B-12 15. Vanadium, iodine, boron and zinc were demonstrated to be non-essential for the growth of H. pluvialis. Higher steady-state cell densities were obtained by a three-fold increase of all nutrient concentrations but a high nitrate concentration remained in the culture medium under such conditions. The high cell productivities obtained with the new optimized medium can serve as a basis for the development of a two-stage technology for the production of astaxanthin from H. pluvialis.