The green microalga Haematococcus pluvialis is known as one of the most useful photosynthetic microorganisms, due to its ability to produce a high value-added antioxidant (astaxanthin). However, this microalga is not easily commercially cultivated because its growth is slow under autotrophic conditions. In this study, the mixotrophic culture conditions were partially optimized using a sequential factorial design. The light intensity and acetate concentration were selected as major factors affecting the algal growth rate. The first factorial design (2(2)) was performed with the light levels of 80-120 mu E m(-2) s(-1) and 40-70 mM acetate. As a result, the optimum conditions were not present within the domain of the first experimental design. An ascent line obtained from the first design showed that the optimum conditions existed within a region of lower concentration of acetate, but similar intensity of light. On the basis of the ascent line, additional experiments were carried out using a central composite design, which enhanced the growth rate up to 0.243 g L-1 day(-1) at a light intensity of 170 mu E m(-2) s(-1) and acetate concentration of 30 mm. (c) 2006 Elsevier Inc. All rights reserved.