Potato starch and both untreated and decationized dextrose syrups were used as substrates for submerged citric acid biosynthesis using a mutant of Aspergillus niger. The same yield of product (80%) was achieved with both syrups and the starch despite having different trace metals content. The obtained mutant was more sensitive than the parent to Cd2+, Mo2+, and As3+, with decreasing yields of citric acid at 10 mg of ions l(-1). Fe2+, Mn2+, V2+ below 50 mg l(-1) and Cr3+, Ni2+, Cu2+ up to 100 mg l(-1), did not significantly inhibit citric acid production.