Aconitic acid, an unsaturated tricarboxylic acid, is obtained by fermentation of an aconitic acid-accumulating strain of Aspergillus terreus or Aspergillus itaconicus, under aerobic conditions, in a carbohydrate-containing medium, followed by isolation of the aconitic acid or a salt thereof. A preferred aconitic acid-accumulating microorganism is the strain of Aspergillus terreus which is on deposit in the Culture Collection of the Commonewealth Mycological Institute at Kew, Surrey, United Kingdom, under Accession No. CMI CC 281924.