THE mycelial mats of Aspergillus niger and Penicillium chrysogenum possess apyrase activity. The organisms were grown as surface cultures on synthetic media, and at regular intervals the mat was analysed for apyrase activity and for ‘7-minutes phosphate’ in the acid-soluble fraction. During the first phase of metabolism, when sugar was being rapidly consumed, the enzymic activity and the content of labile phosphate were high. When autolysis set in there was a marked drop and sometimes even a complete disappearance of both apyrase activity and labile phosphate. Apyrase probably plays an important part in the metabolism of the mould.