IN two recent publications (Jour. Chem. Soc., 200, 3044; 1927) we have presented results which are in agreement with the assumption that the conversion of glucose to citric acid byAspergillus nigerproceeds according to the scheme: glucose → gluconic acid → saccharic acid → citric acid.It was suggested by Franzen and Schmitt (Berichte der Deutschen Chem. Oes., 58, 222; 1925) that the precursor of the citric acid of plants is /3-7-diketoadipic acid, arising from saccharic acid by loss of water. They showed that the ester of the diketo-acid is easily converted to citric acid by alkali hydroxide, a transformation of the benzilic acid type: COOH €¢ CHOH €¢ CHOH €¢ CHOH €¢ CHOH €¢ COOH €”>-2H20 COOH €¢ CH2 €¢ CO€¢CO€¢ CHa €¢ COOH>€¢ + H20OH COOH €¢ CHa €¢ C €¢ CH2 €¢ COOHCOOH This observation strongly supports their view, but apart from the recognition of diacetyl CH3 €¢ CO €¢ CO €¢ CH3 (a .decarboxylation product of diketoadipic acid) in ethereal oils, no further evidence has been adduced in favour of the participation of the diketo-acid in citric acid synthesis either by higher plants or by moulds.Owing to the instability of diketoadipic acid, experiments on its behaviour to A. niger are not yet completed. Meanwhile, it appeared probable that useful indications on this point could be obtained by studying the growth of the mould on adipic acid. One of us (T. K. W.) has shown that A. niger readily oxidises certain fatty acids in the /3-position, and therefore might be expected to convert adipic acid to the ,3-7-diketo-derivative: COOH €¢ CH2 €¢ CH2 €¢ CH2 €¢ CH2 €¢ COOH>COOH €¢ CH2CO €¢ CO €¢ CH2 €¢ COOH. If the mould is capable of effecting a ' benzilictransformation ' the production of citric acid might then be expected. This has now been demonstrated. Potassium citrate has been isolated from cultures of A. niger on the potassium hydrogen salt of adipic acid, and on potassium muconate, COOK €¢ CH: CH €¢ CH: CH €¢ COOK, and characterised as the tri-p-nitrobenzyl ester in each case. In the adipic acid experiment thallous citrate was also prepared and analysed. The muconic acid may give rise to citric acid by addition of two molecules of water forming /3-7-dihydroxyadipic acid, COOH €¢ CH2 €¢ CHOH €¢ CHOH €¢ CH2 €¢ COOH, which on oxidation could yield the diketo-acid and finally citric acid. The conversion of fumaric and crotonic acids in the presence of liver tissue to malic and /3-hydroxybutyric acids (Dakin, "Oxidations and Reductions in the Animal Body," pp. 49-50) indicates the biological possibility of this suggestion.The conceivable formation of citric acid from muconic acid by addition of four hydroxyl groups and formation of saccharic acid receives less support from the biological side. Further work is in progress which, it is hoped, may enable the mechanism of citric ac