1. Octopine dehydrogenase was purified 120-fold from adductor muscle of the fresh water bivalveAnodonta cygnea L. by gel filtration on Sephadex G-100 and chromatography on DEAE-Sephadex A 50 and hydroxylapatite. 2. In comparison to the purified enzyme from marine species the final preparation of the octopine dehydrogenase fromAnodonta showed similarK m (1.0 mM for arginine, 0.4 mM for pyruvate, 1.2 mM for octopine) and pH values (6.3 and 10.1 respectively) and a similar molecular weight (40 000 Dalton). 3. During electrophoresis on standard polyacrylamide gels octopine dehydrogenase showed a multiple band pattern which is due to isoenzymes. 4. Alanine, lactate and succinate had no effect on the enzyme activity. Octopine, however, inhibited it strongly (Fig. 4) 5. Thede-novo synthesis of octopine in isolated adductor muscles from14C-pyruvate was not significantly different under aerobic or anaerobic conditions.