The effect of culture conditions, pH, carbon sources and nitrogen sources on the production of docosahexaenoic acid, the fatty-acid profiles and the secreted acids in the broth by the oleaginous microorganism, Schizochytrium sp. S31, were studied at 30 degrees C with orbital shaking. Docosahexaenoic acid was the main omega-3 polyunsaturated fatty acid produced. The monosaccharides, glucose and fructose and yeast extract were suitable carbon and nitrogen sources, respectively. The fatty acids in the organism and the secreted acids in the broth by the organism were changed dynamically during cultivation. Schkochytrium sp. S31 could accumulate lipid up to 40% of biomass and 13% (W/w) DHA content of lipid at pH 7.0. The more productive culture conditions for docosahexaenoic acid production by Schizochytrium sp. S31 were an initial pH of 7.0 with 20 g L-1 glucose and 0.4% (w/v) yeast extract in 4-day incubation. (c) 2005 Elsevier Ltd. All rights reserved.