Galactaric (mucic) acid is a symmetrical six carbon diacid which can be produced by oxidation of galactose with nitric acid, electrolytic oxidation of d-galacturonate or microbial conversion of d-galacturonate. Both salts and the free acid of galactarate have relatively low solubility, which may create challenges for a microbial host. Galactaric acid was most soluble at pH values around 4.7 in the presence of ammonium or sodium ions and less soluble in the presence of potassium ions. Solubility increased with increasing temperature. Production of galactaric acid by Trichoderma reesei D-161646 was dependent on temperature, pH and medium composition, being best at pH 4 and 35 A degrees C. Up to 20 g L-1 galactaric acid were produced from d-galacturonate using a fed-batch strategy with lactose as co-substrate and both ammonium and yeast extract as nitrogen sources. Crystals of galactaric acid were observed to form in the broth of some fermentations.