Solid state fermentation was carried out to compare efficiency of acid, alkaline and urea pretreatment of sugarcane bagasse for production of citric acid using Aspergillus niger ATCC 9142. Plackett-Burman statistical design was used to evaluate significance of variables. Pretreatment of bagasse by urea was known as the most influential treatment to increase citric acid production (137.6 g/kg of dry sugarcane bagasse and citric acid yield of 96% based on sugar consumed). Finally, up scaling was achieved to a 20 L solid state fermentor in which humidity was constant in gas phase and urea-treated sugarcane bagasse. The produced acid concentration and yield in fermentor was 82.38 g/kg of dry substrate and 26.45 g/kg day, respectively. (c) 2008 Elsevier Ltd. All rights reserved.