The production of citric acid from carob pod by Aspergillus niger in solid-state fermentation was investigated. The maximal citric acid concentration (176 +/- 4 g kg(-1) dry pod), biomass dry weight (30 +/- 0.7 g kg(-1) wet substrate), citric acid yield (55 +/- 2%) and sugar utilization (64 +/- 2.5%) were obtained at a particle size of 0.5 mm, moisture level of 65%, pH of 6.5, and temperature of 30 degrees C. The addition of 6% (w/w) methanol into the substrate increased the concentration of citric acid from 176 to 264 g kg(-1) dry pod.