A novel glucose dehydrogenase (GDH) from a marine bacterium Cytophaga marinoflava IFO 14170 was isolated from its membrane fraction. This GDH catalyzes the oxidation of a hydroxy group of glucose, but does not react in its C-1 position. This enzyme is composed of a single peptide with a mol wt of 67,000. The GDH can react under high salinity. The optimum pH is around 8.0, showing typical property of marine bacterial enzymes. Using this novel enzyme, and enzymatic determination of 1,5-anhydro-D-glucitol (1,5AG) utilizing 2,6-dichrolophenolindophenol (DCIP) and phenazine methosulfate (PMS) as electron mediators was carried out. A good linear correlation was observed from 0.5 mM to 4 mM of 1,5AG.