WHILE investigating the nucleotides occurring in the mycelium of Penicillium chrysogenum, Ballio, Casinovi and Serlupi-Crescenzi detected, besides sixteen well-known derivatives of adenosine, guanosine, inosine, cytidine and uridine, several unidentified compounds. The structures of two of them have now been established, one being adenylosuccinic acid, the other the corresponding nucleoside. The occurrence of adenylosuccinic acid in a living organism has not been previously reported. The compounds described were extracted from the mycelium of P. chrysogenum either with 50 per cent ethanol or with perchloric acid, and purified by ion exchange on 'Dowex-1' followed by paper chromatography. The absorption spectra of both compounds in acid and in alkali were indistinguishable from the spectra reported by Carter and Cohen for adenylosuccinic acid. The nucleotide had the same R_F as enzymatically prepared adenylosuccinic acid in three solvent systems. Using the molar extinction coefficient found by Carter and Cohen, it yielded molar ratios of base, pentose and phosphate: 1:1.05:1.07. On acid hydrolysis (1N hydrochloric acid, 5 hr. at 60°), it gave a base and a pentose phosphate in equimolar amounts. The base had an absorption spectrum typical of a 6-substituted aminopurine and was indistinguishable from synthetic 6-succinoaminopurine in five different paper chromatographic systems. The pentose phosphate was identified as ribose-5-phosphate by paper electrophoresis and hydrolysis with 5'-nucleotidase. The naturally occurring nucleoside gave on acid hydrolysis 6-succinoaminopurine and ribose, and has been assigned the structure of 6-succinoaminopurine ribofuranoside.