Alginate lyase from Turbo cornutus acted on sodium alginate and resulted in a rapid decrease in viscosity and a gradual increase in reducing power of the substrate solution. The result of gel filtration of the reaction products indicated that oligouronides were liberated as the final product. The fact that the enzyme attacked preferentially on the mannuronaterich moieties of alginate molecule was demonstrated by an analysis with paper electrophresis of the reaction products after acid treatment, which separated the oligouronides from the polyuronides.