Crude rapeseed oil and post-refining fatty acids were used as substrates for oxalic acid production by a mutant of Aspergillus niger. Both the final concentration and the yield of the product were highest at pH 4 to 5. With a medium containing 50 g lipids l(-1), production reached a maximum of 68 g oxalic acid l(-1) after 7 d. A high yield of the product (up to 1.4 g oxalic acid g(-1) lipids consumed) was achieved with oil and fatty acids combined.