1. Some of the properties of the phosphoglucomutase of Aspergillus niger have been studied in crude extracts of the mycelium. The results obtained have been compared with the data reported by previous workers for crude and for purified enzyme preparations from animal and plant sources and from yeast. 2. The incorporation of trace elements in a basal synthetic culture medium did not have any marked effect on the specific phosphoglucomutase activity of mycelium. 3. Differential centrifugation of mycelial homogenates prepared in buffered sucrose medium showed that phosphoglucomutase activity is localized in the supernatant from centrifugation at 15,000xg. 4. Spores of Aspergillus niger contain a powerful inhibitor of phosphoglucomutase.