A NAD-dependent myo-inositol dehydrogenase (EC 1.1.1.18) has been purified from the acido- and thermophilic red alga Galdieria sulphuraria. This enzyme catalyses the reversible oxidation of myo-inositol to scyllo-inosose (2-keto-inositol). The activity with scyllo-inosose and NADH was ca 75-times higher than with myo-inositol and NAD. At pH 8.0 the equilibrium of the reaction strongly favours the production of myo-inositol. The K m values for myo-inositol and scyllo-inosose were 430 mM and 1.3 mM, respectively. The dehydrogenase is specific for myo-inositol and scyllo-inosose. The enzyme was purified about 205-fold to apparent homogeneity with a specific activity of 63 μkat mg protein - 1 with scyllo-inosose as respectively. The M r of the subunits was 42 000 and of the native enzyme ca 180 000.