The ability of the citric acid-producing fungus Aspergillus niger to utilize corn distillers grains with solubles as a substrate for citric acid production was investigated since citric acid has a number of commercial applications including its use in foods, detergents and pharmaceuticals. Cultures of A. niger were used to inoculate untreated corn distillers grains with solubles, grains treated at 121^oC and 17 pounds/square inch for 20 minutes or grains treated with a low sulfuric acid concentration for 20 minutes at 121^oC and 17 pounds/square inch. The acid-treated grains were adjusted to pH 6.0 prior to inoculation. The solid-state fermentation of the grains occurred for 240 hours at 25^oC. Following processing of the grains, the citric acid concentration was determined. Biomass production by the fungus was also measured. It was found that the fungus produced citric acid on the untreated grains. The highest citric acid levels were produced by the fungus on the grains treated at 121^oC and 17 pounds/square inch for 20 minutes or grains treated with 1% or 1.5% sulfuric acid at 121^oC and 17 pounds/square inch for 20 minutes. Fungal biomass production increased as the acid concentration used to treat the grains at 121^oC and 17 pounds/square inch for 20 minutes was elevated. Methanol addition did not stimulate citric acid production by the fungus. It was concluded that corn distillers grains with solubles could be used as a substrate for fungal citric acid production.