In order to reduce the raw material cost of D-lactic acid fermentation, the unpolished rice from aging paddy was used as major nutrient source in this study. The unpolished rice saccharificate, wheat bran powder and yeast extract were employed as carbon source, nitrogen source and growth factors, respectively. Response surface methodology (RSM) was applied to optimize the dosages of medium compositions. As a result, when the fermentation was carried out under the optimal conditions for wheat bran powder (29.10 g/l) and yeast extract (2.50 g/l), the D-lactic acid yield reached 731.50 g/kg unpolished rice with a volumetric production rate of 1.50 g/(1 h). In comparison with fresh corn and polished rice, the D-lactic acid yield increased by 5.79% and 8.71%, and the raw material cost decreased by 65% and 52%, respectively, when the unpolished rice was used as a major nutrient source. These results might provide a reference for the industrial production of D-lactic acid. (c) 2008 Elsevier Ltd. All rights reserved.