Abstract：The crude glutamine synthetase (GS) precipitated by streptomycin sulphate from Alexandrium catenella was purified after subsequently passing through ion-exchange chromatography, Gel-filtration Sephacry1 S-300 and Phenyl-sepharose CL-4B. The molecular weight of GS was 430 kD ,which consisted of 8 subunits with molecular weight of 55.6 kD. GS purified from Alexandrium catenella belonged to GSⅠ. The optimal pH and temperature of maximal GS activity were 8.0 and 25℃, respectively. The presence of Mg2+ resulted in the highest GS activity, while GS activity was fully inhibited in the presence of Fe2+ and Fe3+.