1. Phosphoglucomutase [α-D-glucose-1, 6-diphosphate: α-D-glucose-1-phosphate photransferase, EC 2. 7.5.1] was extracted and partially purified from an extreme thermophile, Flavobacterium thermophilumHB8. 2. Some catalytic properties of the enzyme were studied. It was found that the enzyme is characteristic on its optimal temperature (75°C) and heat stability compared with those reported for phosphoglucomutases from various mesophilic organisms. 3. The thermophilic phosphoglucomutase was similar to the E. coli enzyme in requirements for glucose-1, 6-diphosphate and magnesium ion, in inhibition by sulfhydryl reagents such as monoiodoacetate or p-chloromercuribenzoate and in its optimum pH. 4. Kinetic studies were made and the catalytic mechanism was concluded to be the “ping-pong” type. Km for glucose-1-phosphate and for glucose-1, 6-diphosphate were determined to be 2.0×10−4M and 1.3×10−5M, respectively.